Hi SNR,
Well, the combo test at four weeks is conclusive according to doctors, so you know that you did not become infected from the sexual contact.
And you seem to know that the fear of getting infected from the blood draw is irrational. That is the problem with having OCD - deep down you know nothing is wrong, but you still have the little "yeah but what if..." machine running in your head. And sometimes you get over one fear only to have it replaced by another.
I would recommend seeing a therapist of some kind. If you see a psychiatrist, they can prescribe medication that will ease your OCD thoughts. I don't recommend that as your only method though. A multi-pronged approach is better. Use the meds in the short run but try other things such as talk therapy, vigorous exercise, meditation/prayer, yoga and social contact to help. OCD never really goes away completely - you will continue to have disturbing thoughts now and then and sometimes they won't go away easily. But, with some work on your part, you will learn the skills to recognize them for what they are and how to stop them from continuously rolling through you mind.
Thanks for the answer. I think I should get on therapy as you have suggested. Its great to see a concerned human trying to help when you're going through a lot. I had the same feeling when I saw the reply to my question on HIV prevention forum. Appreciate a lot.