I totally agree with Peter. One possible cause of the problem is due to hand kneading the dough as opposed to machine mixing the dough. During hand mixing we fold and push the dough so there is a distinct possibility of developing an air pocket in the dough during kneading, as the dough ferments, gas, created by the yeast moves into this pocket and expands it to create the bubble which you have observed. Just pop the bubble and continue in your normal manner.
Tom Lehmann/The Dough Doctor