I've consulted my medical encyclopedia and although there could be numerous reasons as to why you have a sore tummy, without knowing you, anything about you or completing a medical examination my best guess would be one of the following;
- You have eaten something that your body does not agree with and you currently have food poisoning
- You have eaten too much today, ice cream and potato chips are not good for you
- If number two is not an issue, maybe you are not eating enough and the baby is trying to get your attention
- Pain near or around the bellybutton area could be a sign of colon cancer, you may wish to consult google for further information on colon cancer
- Your unborn child is bored senseless, get outside and do something other than posting on here
- You are in labour, which would not be good

Lastly you may wish to consult a trained medical professional its easier to diagnose a problem if when have something other than, I have a sore tummy to go on.