If it's on places exposed to sunlight, it could be Polymorphic Light Eruption. It's not sunburn and it's not exactly allergy to sunlight, but it is triggered by the sun. Mine started in exactly the same way - fine for a few days, then all of a sudden it came up - blistered as well in places, most unpleasant. You did the best thing keeping out of the sun - it usually makes it worse. It responds differently in different people, but it tends to be the same every time it recurs in any one person. My mum gets it every single time she goes in the sun and uses total sunblock on areas where it occurs. I get it every spring, then gradually as the summer goes on it gets less until by August I can go out in the sun with no problems. Some people stop getting it, other people have it permanently - Mum's had it for 30 years. What to do about it - you may find that gradual exposure to UV light (eg sunbed) in spring or before holidays may help. Mum also takes antihistamines which help, I've not tried them. It's not a sign of anything serious but it is a nuisance. For further information, type "polymorphic light eruption" into www.google.com - there is a lot of information about it out there.