I went fishing in a colder spring day wearing only a t-shirt. it was really cold. now my chest is congested for 3 days. no fever, what could it be?

Chest congestion: I see you are young and on amoxicillin. First the cold weather fishing is not necessarily linked. You likely have a simple bronchitis, presuming you are not asthmatic or have other medical considerations. Since most bronchitis cases are viral, they usually do not require anti microbial therapy in an otherwise healthy person. Supportive care with expectorants is in order. Amoxicillin? Prob not.