Are there cases where strep throat cause immune system to attack heart but it is very mild to be considered rheumatic fever?

Doubtful: If you have confirmed strep throat, the treatment of amoxicillin or azithromycin would protect from rheumatic fever, scarlet fever, etc. There are other conditions that can cause systemic symptoms of fever and shortness of breath (Lemierre's syndrome). If the process was a viral syndrome e.g mononucleosis (which can cause an exudative tonsillitis) the fatigue, sweats would persist. See a doctor.