Review her diet and the amounts of foods she's eating and cut down or eliminate starchy foods, meats and cheeses. Eliminate any junk food from her diet as well. Concentrate on whole grain breads and cereals, fruits (but not bananas as they are constipating) and veggies.
Serving sizes for a child her age are much less than for older children and adults so you may have to cut down portions for her. Too much food can overload her delicate intestinal system and cause constipation.
The following are serving sizes for a 1-2 year old:
Milk/Dairy: Servings: 16-20 ounces (2 - 2 1/2 cups total) of milk per day, so if she's eating cheese and yogurt you'd cut down on fluid milk amounts.)
1/2 -3/4 ounce of cheese = 4 ounces of milk, while 1/4 cup of yogurt = 2 ounces of milk.
Fruits and veggies: Servings: 5 or more per day. A serving size = 1-2 tablespoons - Pureed, mashed, or cubed.
(100% juices made with fruits and veggies that offer full servings of each can be substituted here, and the liquid will help unbind her.)
Whole Grains: Servings: 3-4 per day. Serving sizes = 1/2 slice of bread, 1/4 cup of cooked cereal, 1/4 cup of dry cereal. 1/4 cup of pasta, 2-3 saltine crackers, or 1/2 tortilla.
Non-dairy Proteins (meat, fish, beans, eggs): Servings: 2 per day. Serving sizes = 1/2 egg, 2-3 tablespoons beans (i.e. black, pinto, edamame, etc...), 1 tablespoon peanut butter, or 1 ounce of fish, lean beef, pork or chicken.
If she's not drinking water every day start giving it to her. It helps the fiber she takes in through her diet work better.
And, make sure she gets plenty of physical exercise, which also "keeps things moving." Running and jumping help a lot to avoid constipation.
I used all of the above suggestions with my now 26 month old, who no longer suffers from constipation, and he had bad episodes :-/
I hope she finds relief soon!