It’s difficult to know what could be causing the symptoms you are having. It’s possible it could be STI related or maybe it is completely unrelated to the sex you had.
In terms of STI related I would get another STI test done. If you had a STI/HIV test at 2 weeks it may not have been enough time to pick up some of the STI in a test. Make sure that they check for syphilis as well. It would also be good if you were able to tell the health provider you are seeing that you want to make sure it is not STI related. Some conditions like scabies can cause itching, be passed when having sex but currently have no test to check for this, you need a health provider to do a physical exam who has experience with scabies.
If it ends up not being STI related and the health care professional you see is unsure of what is causing the problem you could always get a referral to a dermatologist. Dermatologist specialise in skin conditions like you have described. Let us know if you need help finding a health professional in your area.
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