It takes a long time for young girls to become "regular". My sister went through this up-and-down kind of cycle and my mom was really worried b/c I didn't (oldest). The doctor assured her that she would eventually even-out and she did.
As long as she's not exeriencing any abnormal symptoms I wouldn't pull her out of camp. Aside from swimming, she should continue to enjoy her time away. Can they give her anything for the cramps? If so, sign consent for that one and let her have fun this last week!