Repotting your christmas cactus and giving it fertilizer can cause problems. New soil will always have a charge of nutrients to feed the plant and adding fertilizer with new potting soil may be too much fertilizer. These plants are able to store large amounts of water, but once that storage space runs out, the plant will literally fall apart; roots rot and cell walls rupture. All you can do at this point is monitor the watering and lighting closely. You Christmas Cactus doesn't look too bad and will probably pull out of this with the proper water management.