If she has severe constipation, increasing her fiber will likely only make it worse. I know; I did that with my daughter, and I actually had to take her and have a colonic done in order to help clean her system out (she was that compacted).
Try giving her some CALM to drink. It's a magnesium powder that you put into water or orange juice (I'd try the orange juice to hide the flavor; if you get orange-flavored CALM, it should work well together) that will gently stimulate her bowels and make her HAVE to go. Both my daughter and I use it if we need a little help going (we both have suffered from constipation our entire lives).
CALM is a natural way of taking care of bowel movements without resorting to pharmaceutical drugs like Miralax (which we've done for years as well with my daughter. CALM works a lot better). It can be found at places like Whole Foods, Trader Joe's, local Co-Ops, or bought online from places like vitacost.com. It comes in a variety of flavors; we stick with orange.
If she has trouble going BM--it hurts because she hasn't gone for a while and the BM is now dry and compacted, which it probably is--try putting some Vaseline around and inside her hole; that will lubricate it and not only protect it from fissures but should also make it a little easier to go.
Also, make sure she's drinking lots and lots of water. Constipation usually is a result of the body being dehydrated. My daughter and I are constantly drinking water.