This post is not intended to alarm you. I just want you to be aware that young pitchers at your daughter's age who are going through the maturity process sometimes get into issues with growth plates in the shoulder.
This could account for the drop off in speed and may not be because she's learning different pitches. You need to ask her if she's having any kind of shoulder discomfort at any time during her lessons.
One of the things that I do with my students is I put them in front of a radar gun about once every three months. This allows me to monitor their speed over the course of their development. There have been a couple of times when I've noticed that there was a drop-off in speed from one three-month period to the next. In almost every case where do the measurement I will see a rise in speed on both maximum and average number of pitches. Basically what I do is have my pitchers throw 10 pitches. I did record the highest and the average.
For the two pitchers who lost speed in both cases they had shoulder issues but did not tell me or their parents. In both cases the orthopedic doctor who examined them determined they had growth plate issues with their shoulder. It took both of them approximately 6 months to recover. The good news is we avoided a long-term injury. The bad news is because these girls are so competitive. They do not want to tell you they're having problems.
In both those cases the girls were about the same age as your daughter. Now this may not be the situation with your daughter , but I just wanted you to be aware this is a critical time in her development as a pitcher. If she is having health issues you don't want to take any chances with her shoulder or arm.
Good luck with your daughter and development of her pitching.
