Call your doctor. It's probably completely fine, but you freaking out and being really anxious won't help you relax and get some rest! I called my doc a bunch from 30 weeks on, and the nurses were always very understanding!
That being said, I went through this same type of cramping and mild contractions for a good 10 weeks! They always told me the same thing:
1. Drink 3 large glasses of water.
2. Lay down.
If your belly doesn't chill out, then come in immediately. My belly always chilled out, though! I bet you're doing too much and/or working too hard! And drinking too little water? Take the day off, and put your feet up. (If possible. Is that even an option with 3 kids???) Drink a ton of water, herbal tea, and/or juice. Relax, listen to some good tunes, and just connect with yourself and your baby again.
You may be one of "those" moms-to-be, who just have an uncomfortable final few months. I was too! But try to rest today, and see if it gets any better.
And call your doc! :) Good luck!