What Causes Strange Bleeding Just After The Regular Menstrual Cycle?

hey, im 17 and i have had strange bleeding today. im worried as i felt sick and had a small period pain but it only 2 weeks after my last one. the blood has been very dark, more brown than red and there hasnt been any as i have been to the toilet unless its on the tissue. there was some of what i think was lining in the toilet before and something i couldnt make out but was a kinda white/see through colour. there is nothing in my pad now and the blood was thin. there only seems to be slight bit on the tissue after i have been to the toilet and it is now very red. is this an early period that will pick up or do you think it could be something else? please....i really need help.