What is it?

I am probably showing how ignorant I am but I have never seen this type of thing before. It was about 6.30 tonight; the sun was behind clouds to the left to where this occurred. Those clouds were quite heavy ones whist the light show was coming from more wispy cloud types. It was like a vertical rainbow in a cloud. It remained there for about 20 minutes, as the clouds moved it grew or disappeared but stayed in about the same spot.

Presumably it was caused by sunlight reflecting off droplets in the cloud but if so why is it not more common.
Anyone got any ideas?