hi..Thanks for the query..
Overexposure may cause sunburn
reaction,which should heal with topical steroid
creams in 1-2 weeks depending on the size of the blister
.You might have tenderness /pain during that time.A course of antibiotics is sometimes required to prevent superimposed bacterial infection
,again depending upon the severity and the extent of area affected.Since you have mentioned 6 large bumps on the sacrum,i think you should consult and get treated accordingly.Sacrum is also one of those areas which take more time to heal due to repeated friction resulting from lying down while sleeping or sitting..