If you've never experienced this before then make an appointment to see your doctor. It is it's too painful to wait go to an ER. I don't want to frighten you, but it's not unheard of for younger people to have heart issues.
Costochondritis is inflammation of the soft tissue between the ribs and it can hurt badly. But mine has never pulsated or gone into my arms. And yes, many with fibro also suffer costochondritis. Hugs love & prayers you get some relief soon!🙂❤🙏🌼