Venu 3 Heart Rate Changes

I’ve been walking and tracking my HR using this Venu 3 watch for the better part of 2 months. My Average walking HR has been between 120-135 depending on my pace. The last 3 days, it’s shown 145-160, my pace hasn’t changed at all, distance hasn’t changed and I don’t feel like it’s been any harder on my body, no signs of the increased HR. I walk like 4-8 miles a day and haven’t had any issues until recently. Does anyone else have issues like this?