I agree with ice and not heat. Heat causes inflammation and your back is hurting because of inflammation...so you definitely don't want to cause more. Ice will reduce the inflammation. Also, youngliving.com has peppermint oil priced REALLY well (it is THE more pure on the market). I've found that filling a tub with warm water and adding a few drops of it does wonders. It causes your skin to feel cooler and reduces inflammation. I suffer from inflammitory arthritis (Psoriatic Arthritis...it's autoimmune like Rheumatoid Arthritis, not from wear-and-tear like your grandma may have) and fibromialga which is, all inflammation. I needed a better saddle, so I picked up a Wintec 500 w/ CAIR. Not only did it make all the difference w/ my horse, but it's helped my back too. And, soaking in the tub w/ peppermint oil in it helps greatly too. I got my saddle on ebay brand new w/ tags for $275 w/ free shipping. Just throwing that option out there as something to consider if/when it's a possibility.
Finally, I would certainly check on your physical strength as well as your riding. Maybe someone can take pictures or video of you riding for you to see yourself ride? If not, is going back to the old trainer an option? Just throwing ideas out there. Good luck in finding a solution. I know how it is to hurt, and I don't wish it on anyone.