Woke Up With Welts On Skin And Rashes Under Armpit. What To Do?
Rash developing overnight can be either due to insect bite or an allergy.
As you mentioned there is no itch, insect bite can be ruled out.
Most probably you have developed an allergic reaction to the linen.
It is quiet common and is called as cloth dermatitis.
Apply a mild steroid lotion like hydrocortisone and take anti histamine tablet. You will be fine.
Let me know if you have any other doubts.