Why is my urine somewhat thick?

I am a 17 year old male, and every so often, (about once a week) I notice that my urine is a little thick. Not all of it, but before I flush, you can tell there is an inconsistency (like oil and water) and just 5 minutes ago, I was finishing urinating, and I noticed that it was almost like my penis was drooling. I know that this can't be serious or dangerous, but I'm just really curious. I haven't masturbated in a week now, and I have not had sex yet so it can't be an STD.