I Am A Homosexual Male Who Is Usually The Reciprocating

I am a homosexual male who is usually the reciprocating partner in anal sex. About two weeks ago I had unprotected sex and this past Saturday I started having diarrhea (yellowish in color). Which by the next day it feels like all around my anus is raw and rashy) although it does not actually look sore. Do you think it could be an STI and how easy is it passable to other partners.