Leaves fall over and get squishy and smell bad.
Growing Calla Lilies in Containers
Certified GKH Gardening Expert
You may be overwatering your plant.
You need to check the soil before watering. Soil should be moist but not soggy.
Well draining soil with an adequate draining pot is important.
If the bulb has all ready began to rot (the smell may be the biggest clue) it may be best to throw away.
Here are some links with more information.