Upset Stomach after Exercise

My Problem:
I am a competitive cyclist on a semi-pro cycling team.  I find that when I'm riding at an intensity greater than 70% V02MAX, which is the effort I can sustain for one hour, that after about 10-15min my stomach starts to get very upset.  It hurts so much that I normally have to get off the bike.  For the next 15-30min the pain will increase and I'll start to have yellow diarrhea for the next 1-3 hours, at this point the pain subsides a bit, say 3 out of 10 on the pain scale.  This diminished pain sticks around for anywhere between 5-14 hours and I'll still have bouts of diarrhea, though not as bad.  Additionally, after the diarrhea, my anus feels like it's on fire and I feel most upset around 1 1/2 - 2 inches below my belly button, making me think it's an intestinal issue.

What I've Tried:
I've tried varying my diet and keeping a food log, but so far haven't found any foods that are the issue.  I've also tried riding on an empty stomach, full stomach and half full stomach.  I've tried drinking straight water and sport drink.  The only common factor I have is the cause, the symptoms are easily repeatable.  

Please help!  This is becoming a debilitating issue.
James

[Ratnakar Kini, MD]

Hi,
Excess exercise can cause diarrhea.
This could be due to stress , shifting of blood from the intestine causing pain and diarrhea.
If there is underlying IBS, it may get worsened.
Consult your Gastroenterologist for evaluation and treatment.

[hammonjj]

I forgot to add, I also have hypothyroidism, if that matters at all