K.C.
Hi, G.! Call your OB/GYN and make an appointment. It is likely that even if this was the same pill you were on prior to pregnancy and it worked fine, it may not work as well for you now. After giving birth, your hormone levels are not the same as prior to pregnancy. If it isn't the same pill, it just isn't the right one.
You will probably want to use a second form of birth control until you have been in to your gynecologist and see what the doctor says about it. You probably will need to change your pill. I have had to change my pill a couple of times over the years.