pain stabbing in tip of penis also burns it comes and goes

i recently had unprotected sex with my girlfriend and while she is having no symptoms i am getting a stinging burning stabbing feeling in the tip of my penis to about halfway up that lasts anywhere from 30 mins to a few hours, it comes and goes but with such severity that it is almost unbearable. i am wondering if it may be an std , a bladder infection or a UTI i am VERY concerned and i NEED help asap, it is making she and i crazy with stress and worry.PLEASE SOMEONE respond asap, at first i thought it was dried up semen but now i am not too sure about that....any ideas or knowledge would help very much thank you.