it is very common to have unpredictable bleeding with the depo provera
. even if you seem to have a usual pattern on the depo, it can change after having a baby. the pattern that you describe is almost certainly just a side effect of the depo.
most likely, if you stay on the shots, over time your periods usually stop totally, but that is not always the case. if it becomes very bothersome, you can talk to your doctor about possibly trying a different method of birth control. however, it is usually not that bad.
I hope this helps, please let me know if you have any more questions.