sunburn can cause blistering and extreme swelling. (speaking from personal experience). Don't be surprised if it takes 10 days for the swelling to return to normal.
Thank you guys soooo much, my "thank you" button is broken or something. I appreciate everyone's kindness. I keep thinking the sun burn thing too but the welts are huge and my feet are so swollen. Normally, I never burn and do keep my feet buried in the sand as much as possible but I run after three grandsons too. We have been back in MO since Saturday and never were around any vegetation of the poison ivy sort. Surely it will be better tomorrow, am barefoot at work, walk in wearing the biggest softest flip flops ever, really cute for an old gal! Thank YOU!!!!