"I filled a bottle of lotion with instant tanning cream and told my sister I found a really good moisturizer she should try. I watched her slather loads of it on her whole her left arm.
She had to wear long sleeve tops for a week (it was very hot that week)."
If it is really old and rusted pipes it can happen randomly, but typicaly after peaking pressure that will rattle the dirt in the pipes loose. Reconstruction where the pipes are knocked can also do this.
I don't think mom actually immediately thought of the exact cause, people...
She just thought something was obviously wrong which was the whole point. Best to move on, future plumbers.