need help with 5 yr old!

Okay ladies and gents, my 5 yr old hasn't been eating well at all lately and her digestive system is paying the price. Every morning before school I give her an on-the-go breakfast (muffins, sausage egg and cheese muffins, pancakes, energy bites, etc). All things that she loves however, it takes all of my patience to get her to eat. I hate seeing her complain about her stomach hurting and sitting on the toilet pooping liquid or puking stomach acid because she's got nothing on her stomach. Even at dinner it can be very difficult to get her to eat. I need help, suggestions, anything please. She use to eat like a teenage boy during a growth spurt and now it's like she's trying to starve herself.