What Causes Yellow Stains In Underwear?

I am 20 im 5ft i have had previous problems like this before but with more thinfs such as:itchy inside my vagina swollen vagina and a burning sensation when i urine and even when i washed that was about a year ago now i have yellow satins on my underwear for about three months now could that be a sign of STD. the stains are very hard to come out of my underwear sometimes my vagina itches but not always