I Dont Know If Its a Yeast Infection Or An Std Help Help Plz

Iam 18 and ive never ahd a yeast infection.Or atelast I dont think I have. I woke up this moring toa buring in my pelvic area and was on the toilet forever(didnt mean to type in caps) . It burns when I urinate and I have a horrible pain in my lower back ...Also a strange colored discharge. I just got home from the doctors I thought it was a bladder infection but from my urine sample he said it was not. He did a pap test and gave me a perscription to treat a yeast infection (monistat) however I was reading the instructions and it said if u have symptons other than what they list or back pain and a strange discharge it could be something else and to see the doctor.I just did that ...Ive never had a yeast infection and I fear this maybe an std......I hate to think like this beacuse I love my boyfriend but ive been questioning his faithfullness......How can I tell what std's it could be if its not a yeast infection? And what are the symptons?