Me and my partner had unprotected sex, 10days after I spotted exactley a week before my period was due, it was a brown and pinkish colour it went on for a day and a half then stopped. I was 3days late on my period then it came on the 4th day but it wasn't my usual period it was really light for 2days then the other two were a little heavier but not like my usual period. I've also had a little back ache and today I woke up and the muscle in the top of my bum is sore in both sides. If I am pregnant I have worked out I'll will be about 4 weeks please help.

Wait 2 weeks after the "light" period and do a pregnancy test with the first urine of the morning.
Yes you could be pregnant

Thanks for answering my question could that be why my bum muscles are sore?

Thanks for answering my question could that be why my bum muscles are sore?
No, there's a different reason for that.