There's yellow stuff in my underwear and I don't know why. it's not sticky, I've masturbated once yesterday and I've also been very wet around my vagina. My guess is vaginal discharge. But I'm not a vagina expert. :[?

If it has a smell, it could be a discharge resulting from an infection, maybe yeast. I'm not an expert either but if it continues, accompanied by an unusual smell and an itch (not necessary) then I advice you see a doctor. If it's just an ordinary discharge, it's probably nothing too serious cause it happens to me rarely. It might just be your organ trying to flush out metabolic waste. My advice is that you don't rely on your own opinion, it's best to get a checkup and be sure that you are safe.