Sox is a lucky cat to have you caring for her! There are a lot of things that can cause constipation in an older kitty, so I'm glad you plan to follow up with your veterinarian next week. You definitely need to try to identify the underlying cause. As far as the lactulose goes, it is a large sugar molecule that passes through the cat's intestinal tract undigested until it reaches the colon. There, the bacteria that normally inhabit the colon break down the lactulose. The by-products from the breakdown attract water into the gut. This keeps the stool moist and allows it to be more easily passed, thus helping to prevent constipation. If a lot of water is being pulled into the colon, Sox may be feeling thirsty and drinking more to replace the water that is being absorbed into the large intestine and lost in her stool. As long as she continues to eat well and act normally, I think you can wait until your appointment next week. Let your veterinarian know what is going on and follow up on having the blood panel done.
Thank you for your quick and mind easing response. I'll be talking to my vet on Monday.