If this is a mini rose you bought at a store, the planting medium probably holds too much water. Overwatering would cause the leaves to drop. I usually shake off as much of the planting medium as possible and then replant the rose in a potting soil mix with no added moisture crystals. The companies have to deliver these mini roses to stores in a moisture-retaining medium because they can't count on store employees to water the roses at the proper times.
In addition, mini roses offered for sale in this manner usually have been pumped full of fertilizer to encourage lots of blooms. Your use of additional fertilizer may have overfertilized the rose. This also would cause the leaves to drop.
