AF returned? or is it just the pill?

I just had my 6 week checkup and have been prescribed the progesterone-only pill for contraception since I am breastfeeding bubs. I started the pills about 4 or 5 days ago, and now AF has hit. Bubs is 7 weeks old. I wonder though is this AF natural, ie. I would have got it anyway, or is this to be expected when first starting the progesterone-only type pill?
I wonder because DH and I are very fertile together and I wonder whether we should be taking contraception very seriously at this point. Does anyone know how long till I am 'covered' by the prog-only pill? Is it a month/one cycle like with the triphasic pill?

When I started on the progestrone-only pill within 24 hours of first taking it I got AF really heavy with bad cramps. As soon as I stopped taking it, it all went away. I then got AF back each month.
It's suppose to protect you if you're exclusively breastfeeding, however I'm not sure how much protection it does give if you're very fertile.