do I have a vaginal infection?

I'm 17 years old & sexually active. I have only had sex with one guy, but he has had sex with several other girls. We have never used protection. Lately I get this itch inside my vagina, at first i thought it was because i shaved but its inside not outside and my discharge is very watery. It kinda looks like if I am always "wet". It looks white on my underwear but when I'm cleaning myself I see it a tiny bit yellow. I don't think I have an STD because the last girl he was with she got checked up & she came out negative. I have read online about vaginal infections but it always says its suppose to smell but my vagina doesn't smell. I think it just smells like a vagina.