my bullmastiff bitch who is ten months and had her first season at 7months is mounting my older bitch.is there a reason or is she confused.

I think it's just hormonal - I know someone with two border collie bitches, who usually come into season together, and they mount each other at this time.  Don't bother any other time.
Also possibly as your bitch isn't in season at the moment, she might just be trying to be dominant over the other dog.