Yeast infection?

I started showing signs of a yeast infection a week ago Saturday. It was burning when I urinated, white discharge and itching. I took the 7 day monostat and my last treatment was yesterday. It went away when I was on it but now it seems to be back again today. My boyfriend was a virgin before he met me and we've had sex 4 times. I don't think it's an STD, because I've only had sex with 3 people before him, all protected, and the last time I was with someone before him was March. Is it normal for them to last more than a week?