Mine do the same. I think it is normal for them to bend over when the blooms start to get old.

Ours are three feet tall and bend over only when it's on the edge. They are straight if they're surrounded by leaves. Maybe it's because we grow them in containers. I have no idea why they do that. It could also be that the stems are too "sponge-y"? [LOL, it just occurred that my choice of words may rub the sick minded the wrong way]

The flowers are pollenated, and are forming fruit/seeds. It's like apple or tomato flowers. It is generally best to chop them off then. (Unless you want seeds. Otherwise, it steals energy from the plant.)

James the Calla Collector

They like full light to stay firm and bushy. I agree with James, once the flowers have been pollinated and start to set seed, they will over, often all the way to the ground.
Some growers give them a Bonzi treatment to keep them from getting too tall.

Susan, another Calla collector
Which reminds me I owe callas to a calla-buddy!