They can want to wee more when they are in season, have you checked to see if there is any discharge, as sometimes they lick themselves clean and do not have any spotting around the house. However, I tend to agree with Lankeela that it could be a sign of cystitis, and would take a sample of her wee to the vets (I know this is easier said than done, best way is to get an old baking tray and pop it underneath her when she stoops down).
Demodex is usually due to a weak or weakened immune system, and I would be inclined to use some Fit and Fertile Protect and Rebuild on her for a few months, (it cleared up my b!tches immune problem, and has had some excellent feedback in the dog magazine) it may also help with the bacterial boils she has on her tummy, as the vet wont spey her until she is fully fit.
http://www.fitandfertile.com/shop/misc-product s/protect-and-rebuild-4.html