Tummy ache every time I eat

I was wondering if anyone had any ideas on this. I've been following MFP since last June and am really pleased with how it's going. On the 1st January I started 30 day shred and i've found that since then every single time I eat I get tummy ache immeditely after. Even if it's just a small snack. I've tried eating more slowly, because I do tend to eat fast, but that doesn't seem to make any difference. The pain passes in about 20 minutes, but it's a real ache, like when you've eaten too much, not indigestion or constipation or anything like that. It doesn't seem to matter what I eat (protein, carbs, fat, combinations) and I haven't changed what I am eating in any way. Toilet habits etc are the same (sorry if TMI!)

Any ideas?