Since you have all signs of some infection which could be a sexual transmitted disease it would be wise to consult your doctor as soon as possible. It is obvious that your girlfriend isn’t saying everything to you and since you can’t trust her about this matter you must take care of yourself. Visit your family doctor and let him know about your problem. He will send you to the urologist who will determine what is wrong and he will let you know about treatment options.
He will probably let you know that your girlfriend needs to see her doctor as well. You are old enough to know that having unprotected sex can lead to serious consequences so try to avoid this. Few hours of pleasure isn’t worth of this what is happening to you right now. Good luck.