It's OCD.
I'm really sorry you're going through this, and I wish I had some words to help you. Unfortunately, OCD is a monster that only grows when you feed it, and seeking reassurance is one of the best ways to feed it. I want to ease your stress, but I don't want to feed your OCD...because the latter would ultimately be detrimental to you.
There are two books that I think are great. One is Get Out Of Your Mind And Into Your Life, and the other is Brain Lock. Feel free to PM me any time if you need to talk.
I wasnt there so obviously cant say anything for sure obviously, but considering your history with OCD especially the preoccupation of you possibly getting HIV i would say it is your OCD playing mind games with you. But you can always get a test done.
I also have HIV related OCD. Perhaps you can refer to my question and offer me advice.
About 4-5 years ago I had a similar experience. I was at a party and there was a guy smoking crack. I joked around with him, "oh you're smoking crack, I have to pat you down and make sure you dont have a weapon." I patted down his jacket and put my hands in the jacket pockets. The next day I was convinced he had a needle and poked me with it. I found a little black mark on my finger tip and convinced myself it was a needle mark.
Unfortunately for me, I was not satisfied until I got a negative test result at 3 months. I was not actively engaged in treatment at the time. I'm not sure it would make a difference. I am in treatment now for a number of issues, but I am having a new obsession of contracting HIV while in an alcohol blackout at 1 am in an empty hotel, even though I woke up alone in my room with no evidence of anything sexual.
This HIV anxiety is a very difficult issue, one that I struggle with and do not have an easy answer to. It may help you to confront your fear and accept what life would be like with HIV and how you could still live a productive fulfilling life. If you are not engaged in treatment, I suggest you seek out a therapist. Also, If I may offer reassurance, its highly unlikely someone slipped a dirty needle into your back. Most people are not injection drug users. Also, those that are guard their needles very closely because they need them. I encourage you to talk to a professional. Also I hope you can take comfort in my experiecne and know you are not alone in these feelings. I wish you all the best!