A lot of things could cause this including your riding once you guys warm up. It's possible that after you are warmed up as a rider, there are aids you are giving that you don't realise are going thru.
Another option as well, is to lunge her before you start riding. Regardless of when she starts to wake up during your ride she obviously has the energy to get going so lunging her before you get on will probably be a good option.
I would go thru what is being fed to her as well and get a routine tack check and rule out any health issues