Stomach pain after eating

For the last month or so, more frequent recently, after eating a meal I start develop terrible pains in my stomach.  The pain goes away after 20-30 minutes.  I have not changed my diet at all.  In the past, I have had similar pain after eating just a few items (bananas, some raw vegetables, iceberg lettuce).  In addition, I have had on and off constipation over the last 4-5 months.  What should I be concerned about based on this increased stomach pain (likely gas) which comes on during meals?  Thanks a lot!