Cactus Rot?

I just discovered these black spots on my cactus and was wondering if someone would be able to tell me what they are and how they are caused
I read it might be due to chemicals? As I recently used dish washing liquid in water to get 'repel' some flies (as told by a florist) I thought that might be the cause, but is there a way to treat it and is it harmful?

 

Definitely looks like some kind of rot, unfortunately it could be a big problem.
If you can stop it progressing it will always be a bad scar on the plant.

Is it in the house or in a greenhouse?
The composts looks a bit too water retentive, which is not good and it needs to be moved to a dryer atmosphere

 

Google. cactus rot caused by washing up liquid. Lots of info.