I managed to gain some (make it a bit too much) weight between summer last year and this spring.I knew why very well. but what stunned me was that i only added inches to my upper body. So: bigger "girls", larger wait and also added a bit to the hips . All my clothes fit (some snugly over the "girls", like DUH).
Have you kept any measurements? If so, grab a tape and measure again. You'll figure out in no time where you gained.
Also, cut down on salt and simple carbs and up the cardio.