On August 25 (little over three weeks ago) I had a sexual encounter with a man. I received a blowjob from him and he tried penetrating me, but apparently couldn't get hard enough. So I gave him a handjob and let him finish.
About a week ago, I noticed a painful, deep, under the skin red bump (basically like a pimple, though I'm hesitant to call it that) on my buttock, not near the butt crack area. I picked at it quite hard, hoping to pop it (I know, I'm an idiot), but that didn't work out and I left it.
A few days later an open sore broke out on it, mostly painless to the touch. Looks like this (https://d2dct7y3250e4n.cloudfront.net/ht-staging/user_answer/avatars/754905/large/open-uri20121219-11670-13oa0sr.jpeg?1355979758)
I am worried this might be a syphilis chancre, although it does not have firm borders and is not hard.
Very nervous about this.