What Causes Yellow Vaginal Discharge?

My boyfriend and I had sex last night. We did not use a condom but I am on birth control. Last week, I was on vacation in Florida so when I was supposed to start taking my placebo pills, I skipped to my next pack of birth control so I would not get my period. This morning and all day I ve been experiencing a brownish yellow discharge and I m not sure what it is from and if it is serious. Should I make an emergency appointment with a gynecologist?