My ball python won't poop?

He usually pooped every 2 to 3 weeks, first some nitrates, and then a big solid poop the next day. He has eaten at least 5 mice since the last time he pooped and its been about a month. I'm a little worried because his last movement was a little scary, I didn't know what was going on at first! There was this horrible sound all of the sudden, which at first I thought was my cat vomiting. But then I noticed it was coming from the snake! He seemed to be struggling sort of, I would almost describe it as a convulsion. I thought maybe he was trying to regurgitate something the way he was moving. Then he stopped, came out from under his cave, curled up, and seemed alright. I took his cave out and it was apparent that he had just pooped. It was somewhat solid but seemed sort of half diarrhea also (it actually looked like a bit of it was forced out so hard that it was kind of splattered) and reeked more than snake poop normally does. I didn't think to take a sample to the vet at the time and just cleaned it up. He behaves normally for the most part, though he seems to be a bit off balance once in a while when I'm holding him. He eats just like he always has and drinks water on his own... which is what worries me, because what is happening to all the waste from the mice he's eaten?? Please help!