Mookey - So sorry you are having such troubles. As many on here will tell you, it takes time to sort out auto immune problems - as there are many of them, their symptoms are variable and somewhat different for each person, and there aren't specific lab tests to give a slam dunk diagnosis - many clinical factors are considered as well.
A rheumatologist is your best bet for the diagnosing and treatment of an autoimmune disorder.
You don't indicate what type of surgery you've had, but you mention nausea and abnormal stools - have you considered celiac disease ? GI distress is not always, but often one of the tip-off symptms for that. Could ingestion of gluten or another food source be triggering problems?
The heart racing and palpitations, from my own experience, can be the result of a food reaction - it happened to me after consuming Splenda, for example - but I also have problems with it from time to time for no explanation that has been found. But it should be checked, as it isn't normal. If your doctor isn't listening to you - best advice is to get a second opinion.
Sorry about losing your beloved pet - they become like members of our family and it is a difficult loss. Often auto-immune type illnesses can be triggered by life traumas such as surgery, a virus, or the emotional stress of a significant loss or change. Don't give up - keep looking for a doctor who will listen to you and try to put your information together in terms of symptoms and problems and a time-line showing the changes. Be sure to ask for copies of all your lab reports.
Best of luck - this forum is full of caring and informed people who have helped me immensely when nothing seemed to make sense.