I have been that badly burnt in the sun outside in the past (teenager, long story but the factor 50 promised turned out to be factor fucking 4 and a bottle two years out of date. Lesson learned, supply own suncream, don't holiday with friends who won't let you leave the beach!)...
end of day 1, agony, red, swelling. Day 2, threw up spectacularly, swollen hands and feet, end of day 2 blistering started to appear. Day 2 more agony, swelling, blistering, peeling..
Was left with scars where skin is thinner (cleavage/chest) and where blisters were deeper (across my shoulders).
Took weeks to heal and a lot of disgusting skin peeling. I could not stand to shower or bath for a good week following the burns either, it felt like my skin was being ripped off me.
Ibuprofen, paracetamol and shitloads of water, electrolyte drinks and keeping very very still, indoors, with as little (super soft) clothing as possible.
I would also speak to the shop, yes you shouldn't have been on for 20 minutes but also there is no way it should have been possible for you to do that!