Yellow may mean infection, not necessarily STD, but it generally means something other than yeast. An infection can happen for a multitude of reasons and not necessarily the sex in the lake. It is a bit like getting strep throat - sometimes it just happens. Best to go to the doctor, they'll take a swab and send it to the lab for testing. Totally painless and then you can quit worrying. Although if you really don't want to have a worry about STD's - best to use a condom.
HIV is on the rise in many areas of the US, so it is a real risk. And females are actually more likely than males to contract it if having sex with someone who is infected because we hold the semen and it is more likely for the virus to enter the blood stream. Guys are still at risk, so guys this doesn't let you off the hook, but it does mean that women hold a greater risk. If you've had unprotected sex, you should add an HIV test to the lab culture. Better to be safe than sorry.
This could possibly be a yeast infection but you need to be seen trichamonis infection an even vaginosis can be what's going on ..you won't know until you are seen .the sooner the better before it turns into something worse ..good luck .