I've been told its normal to have your period every 20 days. I'm 24 years old. I've had 2 children within the last 3 years. Could this be part of the reason My periods are so much closer together and have become more painful?

Sure, it could be normal for you. Of course, you could also have endometriosis, or a tumor. The point I'm making here is that if you are in pain, or are having periods that are frequent and causing you extreme discomfort, you need to see a doctor. No one online could answer this for you with any accuracy.