I used to have tachycardia 120-140b/min now i'm having heartbeats below 60, sometimes 50 why? may any valve problem be involved? even if it is mild?

Tachycardia: Would need more history specifically what were you doing To have tachycardia in the past. Heart rate of 120-140 with exercise/fever is not that concerning in a 19 yr old without other symptom or family history. A heart beat of 50-60 is normal especially at rest. If you're having more symptoms, consult with your pcp who may consider a monitor to check heart rate and rhythm.