Ok so the last period I had was July 1st- 8th and I had sexual inter course around the 10- 25th without protection I was due for my period around the 1st of August but didn't start till like the 4th and the first day was spotting got a little heavier then light again lasted 2 and a half days I always have super heavy periods but this wasn't now I have back ache around my ribs and lower back sore boobs and tired could I be pregnant and I'm 21 yrs old.

It is not very likely, sounds like just a light period.

But in about one week, you can do a pregnancy test, use the first urine of the morning