I Recently Was In Mexico And While Sitting In The

I recently was in Mexico and while sitting in the sun I noticed part of my leg was getting much darker than the rest, I did not miss the area with sunscreen because I did not put any on. Throughout the day this spot kept getting much much darker so I got out of the sun. None of the rest of my skin is burned but this area hurts very badly and it is almost purple in color. The next day it also had a blister. I'm not sure what it is, or if it's something I should be concerned with.