The vet needs to identify the type of crystal, because that will decide the course of treatment. There are 2 types of crystals: struvite and calcium oxalate. Struvite crystals form in alkaline urine (pH 7 or greater), so acidifying the urine (lowering the pH) helps dissolve the crystals and keep new ones from forming. If your cat’s pH is alkaline (7 or greater) then definitely try Methigel. It won’t hurt, and it might help. You’ve got nothing to lose from trying.
Struvite crystals are not precisely stones, but if they aggregate together they form stones.
Unlike struvite crystals, calcium oxalate crystals can be triggered by drinking water high in calcium.
Stress can definitely cause this, so try to think like a cat and look for anything that could be stressing her. Does she have access to outdoors, at least on a screened porch? Are you sure there’s no aggression from other cats?