That’s paint reacting to the previous coat or wall,
Happening regularly,
Won’t happen with top quality Matt emulsion normally
Smokers in the house?
If so sugar soap
Or light sanding on all walls
Room damp?
Dehumidifier 
Either way they’ll go down themselves honestly
Don’t panic ( also can happen if you put second coat on before first is dried properly)
It’s a cunt but no panic.
A bacterial fungus has nested between the blocks and will rapidly make its way around the entire house 
. There’s no solution on the internet so you may adopt the timeless TFK default position:
Send the family away for the week.
Check the house insurance policy.
Burn the damn place to the ground…
Did you vigorously wash down the walls with sugar soap before starting? What paint are you using? Farrow & Ball, Dulux and Crown are grand but Johnstones paint is iffy…
I didn’t wash down every bit of every wall. Just washed the obvious dirty bits and the skirtings and door. I normally do a fairly thorough job. Filled any holes with poly filler. Sanded them down and filled any small holes left with ‘painters friend’. Then dusted and hoovered the whole place out.
Had to take out a built in bed. So plenty of holes to fill. But that’s not the area where I have the problem. It was a dry open airy bit of wall.
The lumps were hard the following day. I learned before not to burst them. They were too obvious and had to be sanded down.