But why those thought always there
Ok, that's good you have had the whole STD panel. I would say at this point you have nothing to worry about, at least not as far as STDs go.
Your OCD on the other hand is going to get worse unless you do something to treat it. Testing for STDs makes you feel temporarily better, but then something will happen that will make you feel like getting tested again. You might even start having irrational thoughts about the validity of the tests.
The first thing I would recommend is to stay away from sex workers. Find yourself a girlfriend who you can have a steady monogamous relationship with. Sex is actually much more satisfying when you have an emotional connection with someone you care for.
The second thing I would recommend is to see a therapist in order to get an evaluation of your OCD. They have a number of treatments that are quite effective in getting your OCD under control. If you feel uncomfortable seeing a therapist, there are also a number of self-therapy books available. Two that I like are "Worry" by Dr. Edward Hallowell and "You Are Not Your Brain" by Dr. Jeffery Schwartz.
The third thing I would recommend is just general good health advice: eat a healthy balanced diet, avoid alcohol and other drugs, do things which you enjoy that keep your mind occupied and most importantly, get regular vigorous exercise.
Hi I just read some of your other comments on the HIV forums. It's seems that your anxieties are triggered by fear of becoming infected, even after sexual contacts that are not ways that can result in infection.
Have you felt anxiety over other issues in the past, or is this the first time you have felt this way?
For the first two contacts, since you tested negative, you have zero chance of being infected with HIV (which is what I assume you are worrying about). The latest exposure, receiving oral sex, is a very low risk exposure on your part. HIV has never been proven to be transmitted that way.
However, there are other STDs besides HIV. They generally are not life-threatening, but they can cause some problems if not treated. When you say you got "tested", what tests did you have run?
You mentioned that one of these encounters was when you went for a massage. Have the others been the same (with a sex worker)?