I got sun burned about 4 days ago...now my arms are tan. Nothing happend to my face because the face lotion?

I used have sun block in it. My questioin is... my shoulder are turning into a really dark shade of tan and like the top part of the shoulder where it rubs with the shirt is not tan anymore it's just red. ( I have to wear t-shirts to work so no I can not wear tank tops to work) is it normal and does the top part of the shoulder where the tan got rubbed off from the t shirt stay like not tan anymore after the redness goes away ? Yes I did have spf on ...i guess it was just not high enough. I did not try to tan at all... I had sunblocks on.