I would go with something like "I'm applying to PT school this cycle and need a reference from a professor. I feel like you know me well from office visits, special projects, and volunteer work. It is important for my application that a professor be able to speak to my ability to do well in the academic setting at PT school. In your classes I received (specific classes and specific grade so she doesn't have to go look them up). (Say something about them next....despite me receiving a C in the first class with you, I was able to do X, Y and Z and turn that around. I've worked hard on my study habits since then). Are you willing to write me a strong letter of recommendation for my application? I will provide you with my resume and and also a few paragraphs detailing my work with you (this is where you basically send a draft recommendation that is super specific about all of your work and points the recommendation to cover your weak application areas). I'm happy to give you a call at your convenience to talk more about this as needed"
On a side note, did you double check that your PT schools will accept 2 Cs? Many will take only 1 C in pre req courses.