I experienced the very same thing when I went off the pill...and I thought for certain I was pregnant! Especially after my friend told me about implantation bleeding...and I thought for certain that's what I had! After going off the pill..I had a little spotting during the month (which is what I thought was the implantation bleeding)...then my next period was really light and only about 2 days long. It took a couple cycles for it to become regular again. I started taking chaste berry (vitex) to regulate my hormones..and also started drinking red raspberry tea for cramps during my period.
So, I think it is possible that what you're experiencing is implantation bleeding..but I think that it's more likely that your body is trying to re-adjust your hormones.
I hope this helps!
And good luck if you're trying to conceive!