If you do get your "period" during pregnancy, it's still very different from a period. It'd be just spotting for a day or two and all brown (old) blood.
After beginning birth control, my period went from 7 days and heavy with clots to 2-3 days and light to moderate flow and no clots when I got on birth control. Once, I even had it just last two days and was just spotting. I've been on it for about 2.5-3 years now. It also usually takes me about 2-3 days (sometimes 4) on the sugar/placebo pills before I will start my period.
What you describe with your period sounds like just that happening. The difference in your breast appearance can be from the hormones the birth control has. Some women will notice their breasts get fuller/larger after being on the pill for a while while some notice no change in appearance.
While I don't think you're pregnant this time, I will advise you to be careful and consider using a condom with your boyfriend in case. The pill does not protect you from STDs, and you can still get pregnant if you do not use your pill correctly (forget for several days, miss and double up more than 2-3 times in a month). Make sure you take it daily around the same time so the hormone levels are fairly steady for maximum efficacy. I set an alarm on my cell phone and keep the pack in my purse so I'm sure not to miss.
Anyway, seriously, don't stress it. What you described sounds like a normal period.