I would definitely have a saddle fitter come out, vet too if it’s really quite sore. I’m guessing your vet would suggest a massage or some chiro but I also recommend getting a back on track saddle pad or mesh sheet as they’ll help your horse’s back a lot!
It's best to get a vet before approaching any other therapist. Most cannot work without vet permission anyway, except a dentist, farrier or saddle fitter... all of whom can be called out before the vet. Although it could be her back itself, just about everything in connected to the back and therefore could be another injury that's manifesting as back pain.
The problem with using cooling gel and then stretches is that cooling gel actually restricts blood flow and contracts muscles, and therefore can cause damage to muscles which should be warm before being stretched.
The problem with using a magnetic blanket is that magnets promote blood flow and warmth, and if there's inflammation, it will make this worse. This is why most physios/chiros/massage therapist won't work unless the vet gives the okay. Here you have two opposite modalities of therapy, and both can cause the pain to get worse unless you find out what the source of the problem is.
I recommend buying the Respond Systems Magnetic blanket. My mare had a severe shoulder injury and this really helped her recover. Also, I recommend having a message therapist to help his get his back relaxed. Hope this helps answer your question!
Get the vet out straight away. Massage therapist next or chiro/ physio as vet recommends
If your saddle isn’t fitted then get that done your saddle should always be fitted to your horse
Don’t ride until it’s all fixed