Welcome to Just Answer and thank you for the question.
I am Dr. Sohaib, a Medical Internist (who deals with diseases of all the organ systems of body) and i will do my best to find an answer for you.
Although it is possible, but a single sex episode in a way you described has a very low chance of HIV transmission, less than 1 % and that too if the partner was HIV positive. Here your anxiety is based upon the assumption that he was HIV positive, however, there are equal chances that he might not be.
But, to be absolutely sure about this, get yourself tested for HIV antibodies. Remember that a negative test will alleviate your fears and anxiety, and will also put your mind at ease.
Wish you good luck.
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