Why is my heart beat jumping again?

Hi, haven't posted in months, my history is longitudinal muscle strain from chemotherapy. I've been doing well on medication (ramipril and ivabradine), breathlessness has reduced and arrhythmia too. But, since I've started exercising and with hot weather, my heart feels really jumpy, I have the sensation of holding my breath although I'm not. It's very uncomfortable. Has anyone experienced this /have any advice?

Thankyou.