I had my last period in aug it went from the 15 to the 12 or 13 of sept and had sex on the 20th and 26th could i be pregnant?

You could: Yes you could. Do a pregnancy test to be certain. Any time you have sex you can be pregnant.

Take HPT: First, i would let your doctor know if you were bleeding for almost a month. Second, your sexual activity was after your bleeding concluded. You didn't note if the sex was protected or not. If you don't get your next period on time - recommend a home pregnancy test. Or you can take an early pregnancy test - they may detect pregnancy up to 6 days prior to your anticipated period.