The first thing is stating your expectations. "Here is your bed. It is nap/bed time. You are to rest here until I come and get you. You get one bathroom trip." this really works with my daughter who is now 5. I set the timer for her nap and tell her she can't get out of her bed until it goes off. I make sure prior to this that we have a predictable bed time routine (bath, book, etc), and I give her cues throughout the evening as to what will happen next. Over Christmas she developed irrational fears about snakes/monsters, etc. Her dad gave her a little Chinese medallion to "keep them away".
I will caution you one thing about tummy aches. If she frequently has them following meals, has nasty diarrhea, bloating, or pain, it could be a sign of something more serious, like Celiac disease. (I had this as a child but did not get diagnosed until I was 27). My daughter was traumatized this year by her immunizations, so now if she says somthing is hurt or she is sick, I ask her if she needs to go to the Dr. She almost always says no. I give her a hot rice bag for her tummy. She really likes those. Good Luck!