Brian: as you described yourself, you have a dog that has more than half a dozen times, yelped in pain for *no reason at all*. Yelped as if 'someone was ramming a needle into her bum'. I've never heard of anal glands causing issues for three months... or recurrent crying out over such a long period? It is just very strange. I have never had any dog do that -- including three with SM, one that had disk problems, and a couple of ruptured anal gland experiences, too.
I'd want a lot more investigation, starting with an xray and if nothing was found, a clinical neurological exam. But that's me.
What in the world does your vet think this was? Seriously, anal glands? -- six times in 2-3 months plus the episode when feeding (and if you remember PDE, walking and lowering the head to feed are typical times when SM pain appears).
If this happens one more time I hope you will seriously consider more investigation than just a vet general exam... (not even doing an xray... )? Vets are not great at picking up neurological issues -- that is why7 on average it takes two years for dogs to get a proper diagnosis when they do have SM. Vets are very poor at checking for signs as it isn't their area of expertise.
Meantime we will all hope with you that it is anal glands, and there will be no more issues.
PS aren't you giving them all sorts of fibre and oat bran etc?