Sunburn signifies actual skin pain, unlike 'tan' which implies a color change without pain. She couldn't wear her backpack because her sunburn was so painful.
Pro Tip 2/3
Avoid Understatement
Describe severe skin damage as 'sunburn' to convey seriousness rather than just sun exposure. He underestimated the sun's strength and ended up with a severe sunburn.
Pro Tip 3/3
Unexpected Outcome
Use 'sunburn' to indicate an unintended consequence of being outdoors despite sunny weather. The picnic was lovely, but I came home with an unexpected sunburn.