Please use some punctuation in your sentences. One big run-on sentence is very hard to read.
If this happened after your friend rode, she very well might be telling you her back is sore. Perhaps it was all along, and the added weight just brought the problem to the surface. Get a vet, massage person, chiro or just friend who understands horse anatomy to palpitate your horse's back and see if there is soreness there. If there is none, then you most likely have a training problem.
You just have to see your distance...you don't have to like it.