Just think of it as a "clerical" issue rather than personal. Start with something like, "I am unsure of how long our sessions are, I thought they started at X o'clock and ended at Y?"

She should take it from there and "confirm" that or say it has changed and why or otherwise give a useful response that you can take from there. If she says they are shorter, do some fact checking of your own and say, "So I am getting 45 minutes for $95? or "So you are billing my insurance company for 45 minutes at $120, they are paying $75 and I am paying you my $30 co-pay for a total of $105?" Whatever it is, just state it like you are stating the facts, trying to make sure you are clear what they are instead of in any way judging her/the amount or period of time. You know, don't say anything like, "Geez, Lady! Wish I made $105 for an easy 45 minutes work!"

The client before you (how do you know when they started? You were there for your session an hour ahead?) doesn't count as they may have some insurance or other situation going on. Maybe they pay $50 more out of pocket than you for their "extra" 10-15 minutes, etc. Just state what you would like rather than compare yourself to others and what you think they have. It is not a level playing field. I paid $200 out-of-pocket for a 100 minute session almost 10 years ago. Don't assume your therapist is shortchanging you until you hear what she has to say and know for a fact she is :-) If she says you have 50 minutes sessions from 3:00 to 3:50 and she's been bailing at 3:35, say, "We start on time every week but the clock in here/my watch/the little man in the funny hat outside the window seems to say (keep erring on the side of thinking you could be wrong/would like to be wrong and not find out she is cheating you) we have been ending at 3:35 most weeks?" Find the exact problem as if you were proof reading a document for typos. If you are using your watch and she says her watch says. . . compare watches/jointly time "this" session, etc. If everything comes out the way it is "supposed" to the next couple weeks, you know she has been caught out and is being dishonest or too clueless about it and you probably should find another therapist.