woke up with long, thin, oval shaped swollen area on my arm

Hello and welcome to e health forum.

The description of the elongated, raised skin bump, which occurs only when you wake up from bed, could be due to a injury or abrasion occurring, while you are asleep.

First and foremost you need to examine your bed to check if there are any wood pieces or any nail like projections which may be the cause of your rash like lesions.

For the time being, you can apply ice to the area, to reduce the redness.

Application of hydrocortisone 1 % ointment, available over the counter, in most pharmacies will also be helpful.

If you have any doubts, you can consult a dermatologist, who can examine you and advice you accordingly.