Std??

About 3 weeks ago I had unprotected sex with a guy I later found out was very permiscous!! About a week after I had sex with him I noticed I was having abdominal pain kinda like cramps. Now I have started getting a thick yellow discharge and my leg muscles around my knees are so sore it hurts me to stand up sit down or go up or down stairs???? Also I am still having abdominal pains. What is this?? It has just now got to the point where I am thinking that I have an std. I am leaving monday to go on tour with my friends band!!! I dont want to get out on the road then come down horribly sick and getting progressively worse symptons. I need to go to the doctor but tomorrow is saturday!! Are health departments open on saturdays???