When i first started riding, i had this problem too. I had to eat HOURS in advance and even then it didn't always help.
When i did the STP i was a mess. it's a double century, 100 miles one day and 100 the next. Including the night in between i was up and down all night... When i was done with that ride, I was certain i could do ANYTHING if i set my mind to it; to be able to finish 200 miles sleep deprived, sore and with a totally messed up digestive system...
So maybe you are just doing too much?