Sessions used to be 50 minutes until codes changed and switched to 45 minutes, ironically so what you are talking about wouldn't happen. It used to be a 50 minutes session and then the other 10 of the hour allotted to writing a progress report on the client. Now they are switched to 45 minutes to give the therapist a 5 minute break in between clients, if they are piggybacking clients as you put it.

I would definately bring it up to her. Don't be aggresive about it or anything, just simply say something like, "I know that our sessions are now 45 minutes, and I feel like we are doing really great work together. I have noticed though, that even tho we always end on time, we usually start late. (give example)." I think that just bringing it to the table for conversation will help things. she might not even realize how much of your session is being cut off.