Sorry to hear of the situation, I can understand why you must be confused. Neither tests nor Psychiatrists are infallible and it wouldn't be the first time a Psychiatrist made a poor diagnosis or a test failed to pick up (indicate) an illness. If there is any uncertainty around diagnosis then the only way to resolve this is to seek a second opinion from a senior clinician (or even a third opinion for that matter). Remember that a diagnosis is a persons opinion and as such they can unfortunately vary quite dramatically from person to person.
I would assume that your Psychiatrist believes you may have Bipolar mood instability because the Paxil made you hypo manic. People with a vulnerability to bipolar instability often react this was to antidepressant medication whereas others don't. Again, this is his/her opinion not a fact and while a manic reaction to the medication is common to people with Bipolar mood instability it is not definitive......there are undoubtedly also people who react in this manner but DO NOT have bipolar instability.
By the way, have tried Cognitive Behavioral Therapy for OCD? It is recommended as the gold standard treatment for OCD and you could try a self help program at home for free.
I hope this helps. Let me know if you have further questions or would like me to clarify anything I have written.