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It is understandably a difficult situation after being sexually assaulted. Regardless of how it felt at the time problems may occur later on in life. Hence it is important for you to undergo counseling which will help you to face the issues better.
The risk of HIV transmission via a single episode of anal penetration is about 1.7% per act.
Sexual assault (anal) can result in transmission of sexual infections including gonorrhea, chlamydia, syphilis, Hepatitis B, HIV, trichomoniasis, etc. Hence it is important for you to get screened for STDs. In addition any potential anorectal injuries also need to be examined and treated.
Hence you may consult with your doctor for a examination and seek proper counselling and proper advice.