That sounds exactly like the umbilical hernia I had. I would go to the doctor and get it looked at. Umbilical hernia's can be a dangerous thing in that the muscle tissue near your belly button has torn and a small section of your intestines can poke through that muscle wall tear. This disallows whatever toxins in the tract to remain in side rather than excreted. If that little bubble breaks, you'll poison yourself. Go see your doc and put your mind at ease.
Kirsty,
Have you asked your doctor about this pain? It could be a number of issues including a hernia. If it persists I would definitely make an appointment to see your medical provider soon.