I think you're over-watering. This time of year is the Christmas cactus' resting period, where they prefer cool temperatures and minimal watering. You can water it more in April. You should also water only when the pot is light, not when it's dry to the touch. To know what this feels like, lift up your plant now; I'll bet it feels kind of hefty. Let it sit a week and try again; it should be significantly lighter. When it feels like it's about to float away, or if you see the pads start to shrivel a little bit, water it. When you water, soak it thoroughly and let water drain through the bottom and then toss any water in the saucer after a half hour or so.
A couple of other things to think about:
1-Maybe re-pot into a clay pot. This is anecdotal, but I'm a believer in the semi-porousity of clay helping roots avoid rot.
2-Put a layer of pea gravel or even regular gravel on the top of the soil.
I have seven of these (one is over 30 years old) and I keep them all in an east window. They get a little direct sun in the winter and virtually none in the summer. If you're going to fertilize, do it during the summer only and use a slow-release formula like Osmocote.