Next Choice/ Plan B extra period for 12 days?

I took next choice aka the morning after pill. About 5 days later I started my period 2 weeks early. Ive read this is common with this pill so I wasn't really worried at first. Only thing is it's been a kinda heavy flow, and it's been 12 days now! It shows no signs of slowing down either. Is this normal? It was my first time ever taking a pill.