Has anyone had an early, short period before finding out you were pregnant?

I got my period over a week early. First day I spotted bright pink, second day it was bright red and average flow (light for me), third day it was brown and now it's gone. I'm on birthcontrol so my periods always start the same day every month. I haven't been under more stress than normal or anything. But I am still a teenager so my body is still maturing and changing, but along with that I've had horrible cramps the day I started and have almost ceased, but my lower back is still relentlessly trying to kill me. I had one beer today and have felt nauseous ever since. Sore , headache, exhaustion, the whole nine yards. Could it be implantation bleeding? Because my birthcontrol completely regulates my period