By stomach ache, do you mean like cramping in your upper stomach? That happens to me when I over work myself, so I have to rest and drink lots of water. I've been experiencing this cramping (in my upper stomach, and lower abdomen) as well as back pains, and my midwife refers to them as contractions.
Yes :/ and it's a daily thing for me, too. As long as the pain isn't consistent, or coming every 4-5 minutes, then I wouldn't worry. But, for peace of mind, you can always call and ask your ob about it.