My BB have that problem when he was two months old. From my experience, it is not a problem, you can wait if you found that your BB is still active, continue to eat, sleep and play just as normal.
If you take BB to see the doctor, the doctor also won't give any medicine to you cause your BB is still small and mama milk BB can have no 便便 for two weeks, so it won't be a problem. But keep on making record on frequency of your BB's 便便.