nauseous when I eat after a long run (10+)

Hoping someone can help! I started training for my first half marathon this summer and completed it in September! I really enjoyed it, but once I started doing the long runs, I started getting sick. I feel great during the run and after. However as soon as I eat something, whether it is 1 hour after the race or 6 hours, I feel horrible and end up going to bed. Like the stomach flu horrible. I have not vomited, but as close as I can get. The symptoms last for about 24 hours....but the intensity slowly wears off ( as far as how sick I feel). If you know of any advice or can point me in the right direction, I would greatly appreciate it!!! I am afraid to continue running long distances if this is always the end result. Thanks so MUCH!!!!