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.