Sick after running long distances

julies90

I am training for a half marathon...I've been at it for quite a few months now, mixing in running a few times a week with crossing training and weights.
Anyways, the other day I did a 20 km trail run...Somewhat farther than I have run before. It went pretty well. Felt kinda tanked after 10 km but finished in about 2:08.
So I felt tired but okay right after, but after about an hour I get so sick feeling (stomach cramps, weak feeling, constant trips to the toilet.)
I was sick for about 12 hours afterwards.
I am really not sure why this happened. I am planning on running the distance again in a few days but I really don't want to experience anything similar again.
Has anyone experienced anything similar? Any tips?

Some details:
I ate a protein shake for breakfast, a banana and peanut butter about 20 minutes before run, and a quest bar at 10 km.