Irregular spotting/bleeding

B • 4 kiddos. Newest edition: baby girl born 2/2016
I had my baby February 24, and went on bc pill at my 6 week check up.  My periods have been right on time with my birth control.  Im just finishing up my 4th pack and should get a period next week.  I have had no spotting in between periods until the last couple of weeks.  It's not a lot of blood...but I just don't understand why now and not when I first started the pill would I have irregular bleeding like this.  I'm not breastfeeding.  Anyone go through something similar?  Thanks