I had a woman lay on top of me, with her vagina facing my butt. I didn't feel any fluids there.

STIs including HIV are transmitted almost exclusively by penetrative sexual contact.  In contrast, contact of genital secretions with the skin, most often as occurs with mutual masturbation but as might occur in the situation you describe is a no risk event.  From what you describe there is no reason for concern of for testing of any sort.  I would not be worried.  EWH
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Thanx.