Not to worry you (you are already concerned, obviously, duh), but are you talking about yellowish discharge directly from your vagina, on your vulva,, or just indirectly on your panties? Reason I ask is that if it is on your underwear, it is probably okay, since our vaginal secretions oxidise and 'yellow' outside our bodies as they dry. In that case, you may just have increased vaginal or cervical mucous for some reason. But, if you are talking yucky globby stuff coming out or in your folds (sorry), there are some conditions that cause this that mandate fairly urgent attention. I'm not a nurse, just a student of my own body, which is why I know some of this. btw, sania19 & Knautica are spot-on in their reccommendation to see a med pro...
I wouldn't be too worried, but I would definitely call my ob/gyn and at least talk to a nurse there and then decide if it needs to be seen by your doctor
this could be infection, i think check with your doctor for safe side.