I would talk to my doctor and ask him/her that question. I had a major heart attack back in 2001 and have been on several heart medications since then. I noticed that when I exercized that my heart rate didn't go up. I was concerned and I talked to my doctor about it. He said to not worry about it. Here I am over 20 years later, still exercising and my heart rate still doesn't seem to rise! (And I always feel so much better after I exercize)!

Maybe it means your heart is getting stronger from the exercise. When I first started using a smart watch after my heart attack, my heart rate could go over 160 when I went walking. Now it doesn't go above 130. I'm just throwing it out there; I definitely don't claim to be an expert.