Ridecamp: [RC] Small bumps under saddle

I was wondering if anyone knows what this may be.
After a 50 mile ride, it doesn't happen on 30's or while conditioning, my 
horse
gets small dime sized bumps on either side of his back about 2-8 inches from
the spine. He has always gotten A's  at the vet checks and the vets have said
not to worry. The bumps aren't tender, when pushed they don't hurt him.
I can't detect any fluid in them and they're gone in two days.
I have tried woolback, coolback, cotton, synthetic  pads. Have used different
saddles and there are never any pressure points from the saddles.
I can brush them, scratch them, push them and no response from the horse.
Anyone get these too? Any advice will be appreciated!
Thanks!
Tracy and Bud

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