We both are 18 and I and my girlfriend had condom protected sex 10 days ago and it was the last day of her cycle. There was no ejaculation from my side and the penis was safely taken out. But she is having periods again within a time span of 10 days. That's worrying both of us a lot. Can any one please specify the reasons for early periods?

We don't know if she is having her period or is having mid-cycle spotting, which is common. She would have to be very specific about how much blood and for how long. Her cycle could also just be all off, due to young fluctuating hormones or stress or any number of reasons. It isn't from having sex per se.

If the flow is heavy, it would be suggested that she miscarried. From what you say, she isn't likely to be pregnant. Keep in mind though that condoms do have a fairly high failure rate, almost as high as withdrawal alone. 'Rates' of pregnancy for both those kinds of birth control are just approximate because there is no good way to do studies.