That's normal. The yeast release CO2 and alcohol when they eat the sugars. When you bottle carbonate (as opposed to forced carbing in a keg) that's where the carbonation comes from.
I've got a 5 gallon batch on the go right now to which I added 1.5 kg of frozen and thawed blueberries in the primary. I'll probably have to add more blueberries in secondary to freshen up the flavour.