Why can't I lose weight?

I am a 20 year old runner and I have been trying to lose the weight I have gained. I was 125 before winter and 132 by spring break (too much snacking during winter break). I run 3-4 days a week, alternating intervals and 5-6 mile runs. I also do strength training 2-3 times a week and yoga once a week. I am currently eating between 1300 and 1500 calories a day. No matter what I do, my waist and thigh measurements are not going back down. I have tried monitoring my intake with weight watchers but it hasn't been working like it used to. Am I working out too much? Am I eating the right amount? Any help would be greatly appreciated!!