Lump on horses back after riding?

[Akskdjwndkh]

Hi all, I have a horse that has started to get a small lump at the base of her withers (on the left side primarily on her spine).
I have had the saddle fitter out and we are going to try some different saddles. My issue is this lump seems to be coming up in all of my saddles (some fitted to a small cob).
We went team chasing and it was as big as a golf ball, though normally the size of a big pea.

It’s squishy and goes after a hour of riding. She also doesn’t show any signs of pain or discomfort whilst riding or if you poke and prod it.

Any ideas of that this is or how to help?

Horse is a 5yr old Connie.

 

[Elia]

Is it there the size of a pea before you ride then gets bigger once she is ridden?

 

[MidChristmasCrisis]

Our youngster gets a lump sometimes on one side of her spine under her saddle, it goes down quickly, isn’t painful and we think perhaps a particular saddle pad might ruck up a little and cause it. Our saddle fitter isn’t concerned about it but I’d be interested in wider views about it too.

 

[CastlelackSportHorses]

My lady gets this too, always on one side and never bothers her. Only gets to roughly pea sized.