I am 26 years old, and my period is 9 days late. how likely is it that i am pregnant? or is there something else that could be causing this?

Depends: Depends on how regular your periods are. If you are almost always a 28 day interval and you were sexually active about 3-4 weeks ago, it is definitely likely. But it really depends on how regular you are. The more regular the more likely. Regardless, suggest you get an over the counter test and check first thing tomorrow morning.

If sex in last 3-4 w: Your profile says you're 47 and the question says 26. That would make a difference but assuming you are 26 and always have regular periods and you've had unprotected sex within the last 4 weeks then yes, i would say it is likely that you are pregnant. That's the first thing any doctor would want to determine so i suggest you go get a pregnancy test right away.