I used to burm fairly easily when I "forgot" the sunscreen/sunblock. This past summer I was outside for hours on end and never turned pink, much less get a full on burn. As far as I know and this might be coincidence, the only difference between this summer and any other is my vitamin D3 level...
A couple winters ago, my level was measured at about 8 ng/mL. Doc had me supplement with D2 for four weeks and got my level up to about 35. By September, it was back down under 20... And I know I got a bit burnt that summer.
Just after that summer, I started supplementing on my own with D3 and got my level up to 115 or so. The following (this past) summer I was outside all the time and got browner than I ever had in the past. It was amazing.
Something to think about if you burn easily...