If she has been constipated for a few days, she will really need to get into a vet, as a enema will be necessary. The feces can get very impacted, and stool softeners/laxatives will not work.
In fact, I almost never recommend laxatives in pets as they can cause horrible diarrhea and bad abdominal cramping.
What I would say is you could try fiber supplementation. Now I just realized you wrote this in the cat category, but you said you have a dog.
So, if you have a dog:
1. You could give a teaspoon or two of canned pumpkin (not pumpkin pie filler) with each meal, or a tablespoon if a medium to large size dog. Might help some.
Or, you could use a teaspoon of metamucil.
2. Encourage more water intake. Adding a little canned food to her water or some canned tuna juice can stimulate drinking.
If she doesn't defecate and she has not defecated in 3 days, she really needs a vet.