Hi woodswalker, welcome to the board!
The dose of Doxy you are taking is not in the treatment range for Lyme disease. The recommended dose is 300-600 mg daily with 400 mg daily often prescribed. Twenty-one days of treatment is not long enougn. From what I understand, an infection treated within two weeks of the bite needs a minimum treatment time of six weeks of the proper dose antibiotic. I am not a medical professional, but since you have not gotten the proper dose I believe you should be treated longer.
Are you aware of the tips about taking Doxy? Do not have any dairy, iron, or magnesium products within two hours of taking the Doxy or it will not be absorbed properly. If it makes you nauseous, eat something substantial and non-dairy before taking it. Avoid the sun. You can get a severe burn in minutes even when wearing sunscreen. Do not lay down for at least an hour after taking it or you can ulcerate your esophagus. This is very painful. Drink a full glass of water when you take it for the same reason. Whenever you take any antibiotics, make sure you take high quality acidophilus capsules at least two hours apart from the Doxy. This helps to replace the good bacteria and prevent yeast overgrowth. The best kind to get are refrigerated and have a high culture count. Most healthfood stores have them.
Why does your doctor want to do a spinal tap? They show Lyme is less than 5% of cases. I have never heard of a Lyme knowledgeable doctor ordering this test to test for Lyme. People who have Lyme should not take steroids. They can make symptoms much worse. I recommend talking with your doctor about tapering down and stopping them. You cannot stop them abruptly. It does not sound like the doctor you are seeing is knowledgeable about Lyme.
Below is a Lyme symptom list. You can have any combination of symptoms.
Unexplained fevers, sweats, chills, or flushing
Unexplained weight change--loss or gain
Fatigue, tiredness, poor stamina
Unexplained hair loss
Testicular pain/pelvic pain
Unexplained menstrual irregularity
Unexplained milk production: breast pain
Irritable bladder or bladder dysfunction
Sexual dysfunction or loss of libido
Change in bowel function-constipation, diarrhea
Chest pain or rib soreness
Shortness of breath, cough
Heart palpitations, pulse skips, heart block
Any history of a heart murmur or valve prolapse?
Joint pain or swelling
Stiffness of the joints, neck, or back
Muscle pain or cramps
Twitching of the face or other muscles
Neck creeks and cracks, neck stiffness, neck pain
Tingling, numbness, burning or stabbing sensations, shooting pains
Facial paralysis (Bell's Palsy)
Eyes/Vision: double, blurry, increased floaters, light sensitivity
Ears/Hearing: buzzing, ringing, ear pain, sound sensitivity
lncreased motion sickness, vertigo, poor balance
Confusion, difficulty in thinking
Difficulty with concentration, reading
Forgetfulness, poor short term memory
Disorientation: getting lost, going to wrong places
Difficulty with speech or writing
Mood swings, irritability, depression
Disturbed sleep-too much, too little, early awakening
Exaggerated symptoms or worse hangover from alcohol
It is very important to see a knowledgeable doctor. Many doctors do not understand Lyme and treat with outdated protocols. No test is completely reliable, and results can vary by lab. Besides Lyme, ticks can also transmit several co-infections including Babesiosis, two types of Ehrlichiosis (HME & HGE), Bartonella, and Mycoplasma. Have you been tested for these?
Are you aware of the Jarisch-Herxheimer Reaction (herx)? Often when people who have Lyme take antibiotics, their symptoms get worse or they get new ones. When the antibiotics kill the bacteria, toxins are released making them feel sicker. It can be scary, but it is a sign the antibiotics are working. Although it can vary individually, many people experience this at the beginning or change of treatment and every three to four weeks. During treatment it is good to document your symptoms daily. One way to do this is to list the main symptoms you have each day with a numerical rating of their severity, from 1-10. Over time when you review this, you can see when your herxes occur and how you are responding to the meds.
If you need a doctor recommendation, you can email me at firstname.lastname@example.org Your infection is still relatively early which is good. Early infections are the easiest to cure. Getting the proper treatment early may reduce the chance of long-term complications. I cannot stress this enough.
It is also important to learn as much as possible. I recommend reading Dr. Joseph Burrascano's 2005 Diagnostic Hints and Treatment Guidelines For Lyme and Other Tick Borne Illnesses at http://www.ilads.org/burrascano_0905.html He is one of the top Lyme doctors in the country, and many Lyme doctors follow his protocols. I also recommend the book "Everything You Need To Know about Lyme Disease (2nd edition)" by Karen Vanderhoof-Forschner and "The Lyme Disease Solution" by Kenneth B. Singleton MD.
Hang in there okay?