"I, not events, have the power to make me happy or unhappy today. I can choose which it shall be. Yesterday is dead, tomorrow hasn't arrived yet. I have just one day, today, and I'm going to be happy in it. "Groucho Marx
Thankfully you're not a girl, because every few weeks we get something similar in the form of cramping ... oh the joy!
But, on a more serious note maybe it is something you are eating. Could be (and I'm reaching here) lactose intolerance. Or just some type of food that isn't agreeing with you. Try making note of what you eat and see if you could notice a pattern that way. Maybe you aren't eating at regular intervals? Could be also you are eating too late at night? Eating too much and not digesting it properly?
But, if this gets really bad I suggest you see a doctor as soon as you can. It's better to go when things aren't so bad, then wait until you are dialing 911 in pain.
but my first semester of college, once a month Id' get a bad pain in my side and throw up. It went away second semester though.
Good morning, son
In twenty years from now
Maybe we'll both sit down and have a few beers
And I can tell you 'bout today
And how I picked you up and everything changed
It was pain
Sunny days and rain
I knew you'd feel the same things...