All birth control can cause weight gain. A change in diet is necessary during use of birth control to try to ensure this weight gain doesn't happen, or happens minimally.
I recommend exercise and a change in diet (even just a small change in diet).
Crunches are great for toning muscle and helping lose belly fat.
Jogging is a great form of exercise...
Cutting out big dinners and PM snacks for salad and fresh veggies (no fruit in the late PM either), and make sure the last time you eat is more than 3 hours before bed.
If after this change in lifestyle, you see no improvement (not on the scale, but via measurements), then you need to talk to your doctor to find out if there are other forms of birth control that won't affect your weight/size, but will help you (whether it be to prevent pregnancy or to prevent period pain-dysmennhorea.)
GOOD LUCK! I have been right where you are!! I used to jog 1-3 miles a day while I was on the Shot, and I was about 140, and I'm 5'1.5"...so I wasn't fat, but the water weight wouldn't go away!!!!!
(coincidentally, after the shot, I didn't lose the weight, and 6 months later, with pregnancy, couldn't keep weight on due to the world's worst morning sickness!!!!).
so, good luck! *hugs* talk to your doctor, he/she will be able to help you figure something out ^_^