I don't know it this is semen or not?

Well I was just playing around with my penis and kind of squeezing the head a bit but suddenly I felt like I had to pee and as I stood up, with my erect penis sticking out, this white-ish / yellow-ish foamy stuff came pouring out just for a second or two. Was this semen? I really don't want to do that again, I feel really ashamed of myself. Masturbation is a terrible thing to do.