Answer:
What birth control such as the shot or the pill does is it shuts off your body and takes over on auto pilot. So, when you are coming off the pill or the shot it takes a while for your period to start up again and especially take a normal frequency. Some women it takes years, depending on how long you were on the birth control and how high the hormone levels were in the shot. You are probably not pregnant, it's just that your body is trying to restart itself. Give it time. If you are really concerned see your gyne.
Without knowing your age or general health/medical history, I can only give you a generic answer for this...
If your menstruation has been chemically halted for a long period of time, it will take some time for your cycle to return to normal. My wife, for example, has not had as heavy a period since coming off the Depo as she had prior to starting birth control. She used Norplant prior to the Depo shots and hadn't had a period in about 8 years. We decided to start our family last year and it took about 8 months for her cycle to flow "normally", but still not as heavily as when she was a teenager. (btw, we're expecting - so she returned to "normal enough.")
If your cycle does not return to a near normal flow in the next few months, call your physician (OB/GYN if you have one.)
It takes 3 months for it to totally get out of your system. I used to take it and it stopped my peiod all together a took a few months for it to get back to normal.
I just want to say be careful. My friend was on the depo provera shot for 2 years and her period won't return. She's been givin many big hormonal drugs to induce her period, but nothing. They are now thinking that she's going into menopause..and she's only 24 years old. They claim it takes 3 months to get out of your system, but it can take over a year for your period to return if you lost it.