Heartrate during workout on Apple Watch

We did IVF in Jan/Feb so I haven’t worked out since our egg retreival, we did a fresh transfer, so it’s been about 3 weeks since my last workout. I do boxing, a bag, not a person... 😉 I have a Apple watch, which I believe is quite accurate, but my heartrate went from 120 to 140 to 120 to 160 then 73 and then 160? I got a hige fright and am considering quitting my workouts and changing to something like just walking with the dogs... Has this happened to anyone else? I’m very worried! Almost 5 weeks now... 🙈