Hmm. Well, it's not a very satisfying answer, but it sounds like something you're eating isn't agreeing with you. First, I'd look at foods you're eating a lot more of. Bananas are typically a treatment for loose stool, rather than a cause, but maybe lay off for a week and see what happens. If you're not eating much fiber, try adding some (especially soluble fiber), in the form of rice, wheat, pureed pumpkin, or oats. If that doesn't work, try vegetable fiber instead. Eating lots of fruit can cause problems for some people.
You might also try resetting your digestive system by skipping a meal or two, and/or trying an over-the-counter remedy.
If that's not working, talk to your doctor. She may be able to shed some light on dietary or non-dietary causes.