You're probably not overreacting. The best way to know if your child has a weight problem for their age is get them evaluated by a pediatrician. They'll check her out and if she is overweight, they'll know the best way to adjust her diet so as not to deprive her of essential nutrients she needs to grow.
If you feel she's sneaking food, don't buy food that is sneakable. If you don't have unhealthy, high calorie food around to sneak it will solve that problem.
Get healthier snacks to keep around. Most humans should exerice at least 20 minutes of cardio, 3 times a week at minimium. This should be fairly easy for a healthy young girl like her. Maybe she'd like dance aerobic classes for children. They're pretty fun. Even if it turns out she's not overweight it's still imperative to instill in her healthy excercise habits that she can keep a lifetime.
Food shouldn't be used as a treat for good behavior. For example, when she behaves well, a promise of a trip for ice cream isn't such a good idea. I feel, using food as a reward for anything instills people to grow up with the idea reinforced that food is a reward when in fact what you want to teach them is that food is simply fuel and that you only need to eat when you're hungry.
Good luck.
My 7 year old daughter's weight 
That is sad. My daughter weighs seventy pounds and she is tall for her age and by no means thin. She is seven almost eight. Time for a trip to your pediatrician. Remeber, it could be a physical problem causing it. Have her checked.
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