Whats wrong with my knee?

About a week ago i was running and the next day my left knee started to kind of hurt. i can walk on it but it just doesnt feel as strong as my right. and sometimes if i am not thinking about it it will buckle under. if i run it reallly hurts. and if i am sitting on a chair and try to straighten my leg out it hurts.. i am wondering if it could be my ACL? what do you think? thank you!