What Causes Spotty Brownish Pink Discharge?

My girlfriend was supposed to start her period yesterday. She did indeed start, but it was spotty brownish-pink discharge for most of the day, and then began to get slightly heavier and turned more blood red. Her first day is generally the heaviest. We only had sex 3 or 4 times last month, and we use a condom 100% of the time, and practice the pull-out method. Could this be implantation bleeding?