Hi Marius and welcome.
I'm sorry to hear about your ailments. I'm sure you are confused as well as fed up. Though I haven't had exactly what you are going through, I had some similar experiences: I was hit hard at 25 1/2 years old as well (though I grew up with juvenile rhuematoid arthritis) with debilitating pain. For a couple of years leading up to that point, it was the strangest thing - it was like my body was just searching for a place to attack. I dealt with infections, sores, roaming joint pain and so forth.
I have since be diagnosed with psoriatic arthritis as well. When I was diagnoses as a child, everyone was baffled as no one (that we knew of) in the family had any form of arthritis. My theory - it is a result of a childhood illness (rubella) that often leaves it's mark w/RA.
The funny thing about psoriatic arthritis is that it is one of those autoimmune diseases that affects more than just your joints. It can make you break out in these painful bugbite/pimple looking things all over the place (they are pre-staph infections, so watch them closely!), your fingernails will look like you were punching them with a blunt pen tip (called pitting), your organs can be affected causing inflammation and infection (I am often hauted by numerous bladder infections), it can cause psorisis outbreaks (obviously), it can cause colitis-type symptoms, and so on.
Swelling, stiffness, warm/red joints.....they are all signs of arthritis, but not always apparent nor at the same time. Nor can you rely on the blood work results.
With all forms of RA, you can have times of remission, flares, and times where you are 'just getting by." Do you have multiple diseases. Possibly. Could all these issues be the result of psoriatic arthritis. I'm guessing, possibly is the answer as well. The azulphidine will help with the colon issues, if you are will to try that again.
I would stick to keeping in check with doctors and getting the right answers and treatment (this can prove to be a trying task, but when you are getting rashes that are turning in to infections like that you don't want to let them go!) You know how you feel, don't let the doctor's treat you like you are crazy, or dismiss you because blood work has come back normal. That really isn't a fool-proof way to know if you have arthritis or not.
I have to ask - when all those rashes and pain started, you never once went to the doctors?
I'm sure someone w/more medical knowledge will be around shortly to answer you. I just wanted to welcome you and let you know you are not alone out there. I hope you find some relief and answers soon. Hang in there!