Hi Girthy Horse Owner
I too have been through this:
1) find a well respected saddle fitter and have your saddled checked; it could be that your tree is too narrow or wide and when you tighten the girth a second time your are driving the tree into his/her shoulders or putting the saddle on their vertebrae, either way OUCH!
If all is well with the saddle fit,
2) have the horse checked for ulcers. You can start a vet prescribed ulcer treatment without the scoping, but why pay $1000 for Gastrogard if you don’t need it? I live in the Maryland/Virginia area, scoping a horse here cost about $200; 1/5 of the price of Gastrogard if the test is negative and it gives the vet a better idea of what could be bothering your horse.
My horse bit me so many times before I understood what he was trying to tell me I felt stupid when I found out it was the saddle. A word to the wise, find an independent saddle fitter–one that does not represent a specific company or brand–you will get an unbiased response from this type of person. I learned that lesson the hard way.
Once you have ruled things out (saddle or ulcers or whatever) then you can add acupuncture or chiropractic work as necessary. I hope this helps!