What Causes Flatulence And Pains Randomly With Sickness?

Hello, I have terrible gas and pains that seem to come randomly and stay for about 3 days. I can't eat right, and I feel sick. I am a runner, I am 5'7" and 121lbs. I have three children under the age of 5, and it seems to have gotten worse after each delivery. I have a sister with Celiac Disease and I have been tested but the results were negative. Any suggestions on what I could do to handle this?