Definitely normal!! I felt physically and mentally drained, and definitely overwhelmed, for the first 2-3 weeks of my 13 week OP rotation. There is a lot to pick up in the OP setting treatment wise, and there are so many different schools of thought on therex and manual techniques that I think it is normal to need a little time to acclimate to the pace and thought-proces of the clinic you're working in.
It depends on what aspects you feel most overwhelmed with, but I found it helpful to look at the diagnoses for my new evals scheduled on the following day, and to then brush up on info for that body area/diagnosis the evening before said evaluation. That way you aren't trying to loosely review too much musculoskeletal content without a context, which can be overwhelming IMO. Ask lots of questions to understand things early on, and make sure you communicate early on if you're having trouble picking up techniques. Good luck! 🙂