I got the Mirena for similar reasons - I'm useless at remembering to take pills and I don't like the idea of hormones being throughout my whole body - the Mirena hormones are localised.
I don't know if Mirena can cause cysts but I read all the leaflets and was cross that my Dr hadn't been more honest about the bleeding and cramping pain as apparently 75% of women experience this. I would have thought twice about getting it if I'd known how bad the cramps would be.
I know what you mean about it being difficult to look after your kids when you feel rubbish. I was lucky that my hubby was at home (he was made redundant so is around more at the moment) to help with the kids or I'd have gone mad. Take lots of painkillers and see your Dr as soon as poss.