Babies and small children do get stomach aches quite often, and if they are feeling tired or anxious they may be more prone to small ailments. In particular, if a baby is under stress this can lead to some stomach pain. However, any pain that seemed to go with stress should be mild, and would be gone as soon as the child was calmer or had had a slept. So if your baby has a severe stomach ache or one that goes on for a while, you should look at his or her symptoms carefully.
If the pain is bad enough to make the baby cry a lot, wakes him up at night or goes on for several hours, you should contact a doctor. This is especially true if there are other symptoms such as fever or a rash, or if she is being sick, has diarrhoea or other stomach-related problems.
You can find more detailed advice here.