Could I possibly have an STD?

Ok, I woke up this morning from a y dream about my girlfriend. So, when I woke up, I had an and, naturally, "took care of it". When I had pre-seminal fluid flowing out, it was a yellowish substance, and not clear like it usually is. Also, when I d, it was slightly yellow tinged, and stung slightly coming out. Nothing unbearable, but a slight sting, which never happens. Now, some background info: I have been dating my girlfriend for 6 months, and we have been ually active for 2 months. We lost our virginities to each other, and I was tested for any genetic STDs before we had . I was clean. I have not been cheating on her, and she is not the type to cheat on me. We have not even had recently (i believe the last time was last wednesday). Also, I was terribly sick yesterday with lots of mucus and drainage and coughing, etc. So I am hoping that may have caused it. This morning was the first time these symptoms appeared, so I am thinking it could have been because I was sick (still am sick a little bit). So, does anyone have any advice on what may have caused this, should I be scared, what to do, etc? I am really worried, and my girlfriend says she doesnt have anything and is worried a bit for me too. Please, answers?!?!