Could she has possibly injured her back outside of cheer practice and is feeling the residual effects at practice due to the extra strain? If that's the case then she should have some notation of her injury from a dr. and a clearance note and all that good stuff.
If it is strictly cheer related I would look out for the "out and around" some of my girls were doing this even with straight backs. Instead of using their legs and pushing straight up when the flyer got into their hands, they used more arms/back and would move the flyer's feet out away from their body and sort of "scoop" her into position. This puts a lot of strain on your back and my bases complained too, until we fixed it.
As for a backspot, is she standing right up against the stunt? I tell my backspots to get their "ta-tas" as close to the bases as possible. If she is standing away from the stunt and reaching with only her arms that may cause some back strain, especially on cradles.
That's all I can think of, hope it helps
