Hi, For A Couple Days Now I've Been Feeling Like

Hi, For a couple days now i've been feeling like my chest is clogged and someone is sitting on it at the same time. it makes it hard to breathe. The doctor i went to told me i had costrochrondritis. which i think is a false diagnosis because i'm not in any pain. i take a deep breath and hold it and it feels like my heart skipped a beat. i don't know if i'm over thinking it but i'm really scared and i don't know what to do. please help.