Is 110 lbs overweight for a 5'1 13 year old girl?

Absolutely. The best way to check your healthy weight is to use a Body Mass Index (BMI) calculator. Here is a good site for that stuff for your age group:

http://kidshealth.org/teen/food_fitness/dieting/weight_height.html#

I know you probably don't want to think of yourself as a "kid" at 13 (I know I didn't), but the weight/height comparison is a little different for teenagers than for adults because of all the changes your body is going through for puberty. Young teens require a higher percentage of body fat than you will as an adult. I took the liberty of entering your stats (height, weight, and age) and you are in the middle of the "healthy" zone. So you're perfect :-)