Yellow/White Substance Coming From My Penis?

After I urinated during the middle of the day today, I thought there was still urine that had to come out so I pushed a little and then this dense white/yellow sticky substance came out. Could it just be a build-up of semen? I'm not sure because I've never masturbated before, but I've had "wet dreams" and that was a while ago. I'm 18. And if this has to do with anything, there was a pain in my left testicle the other day for a few minutes then it passed. And I've never had sex so it is not an STD. Is this just normal?