Is it always an STD......?

Well i had unprotected and the next day i didn't shower( i know, nasty), because i was sort of hung over.. anyways by the end of the next day i noticed a white sort of blister/whitehead looking thing on my under the head of it. It sort of itches and a bit painfull if touched. I know i've had similar situations after before.. i mean if i didnt wash my afterwards it seems it would get irritated, but i never had a blister looking thing. so i am asking is it always a STD if you have a blister there? it's white, but with no redness and there was only one. it itches a little bit and a bit painful to the touch.. and iam uncircised.