You’ve likely got chronic urticaria, and will need to take a doctor-prescribed course of anti-histamines to get rid of the itchy rash.
No cause or trigger is found in 80% of people with chronic urticaria, in case you were wondering. It’s just one of those things that happen, unfortunately.
Good news? It’s self-limiting, and responds very well to treatment (oral anti-histamines).
The dents in your nails sounds like nails pitting (shown in picture below). There are several causes for nail pitting, of which psoriasis is an important cause to rule out.
Do visit a doctor/dermatologist, who can give you a proper diagnosis and the necessary treatment for both the above conditions.
In this instance, I’d probably lean towards visiting a dermatologist as they are the experts when it comes to nail and skin problems.
Cheers
