
I have just been through the last month with this situation... first we tested for uti,bladder stones and all came back negative. My girl is 9,was spayed at six, and just presented with the full flooding.This was occuring both day and night....I have put her on incurin tablets,for post spay incontinence,1mg a day and on the third day she was dry again.We are now on day ten of the meds and I am about to try alternate days and see if she remains dry.Good luck...I know just what you are going through.
In the meantime if you can sew,I suggest making waterproof sheets from a single cotton bed sheet cut into two, and backed in polythene. These make great ,cheap protectors. I also slip a poly waste bag over my dog mattresses,to keep them dry,then cover the lot with a duvet type cover which is easily slipped off and washed.
I also take the dog out at ten pm to be sure she has an empty bladder to face the night ahead.This has helped greatly.