it sounds like your baby is developed an umbilical hernia. It happens quite often, with it's main sign being noticing a small soft lump in your baby's navel. You should confirm this with the pediatrician, of course, but the general practice is to first let hernia repair itself - so most doctors won't do any surgery before a child reaches 12 months.
Exceptions are the situation where hernia continues to grow or is painful for a child. In those cases a relatively simple procedure is done to close the hole and repair umbilical hernia,
Wish you all the best,