I've had the same questions. One doc said I have RA markers and wrote me up as seropositive. Next doc said no, that was a bogus test, and wrote me up as seronegative. Lol. I have symmetrical inflammation and pain in many many joints, and I think that's why they agreed on RA, despite not agreeing on the tests. Sometimes I wonder if I do, in fact, have RA - but whatever I have, it's immune-related, and most of the treatments seem to be used for all the other immune conditions as well, so at this point I'm not sure if it even matters for me. I just really want a name to call it, I guess, but otherwise I'm not bothered.
Ask your Dr what else it could possibly be, is my advice. In my case, initially, my dr suggested it *might* be lupus based on test results, and the meds he gave me could be good for either RA or lupus.
And - be aware that RA can cause a ton of damage pretty darn quick, so they like to catch it early & start trying to damp down the activity. Talk with your dr until you're feeling OK about taking the meds, and report side effects if you get 'em. Hope you feel better soon!!!