heart rate on Garmin shoots up when using Hammer drill

Whilst doing some heavy duty drilling into masonary wall, and used the Hammer drill option, so a huge amount of vibration in my hand..I looked at the watch few hours later and it stated my heart rate was ridiculous during the drilling, about 220. My heart restting rate is about 58, and dosent go much further than about 150 when running fast on treadmill, so its in good shape really. So i can only assume the vibration of the hammer drill gave a false reading?? I dont know but i felt fine doing the drilling and no racing heart or flutterings, so there's no way my heart was at that rate...Dont know if anybody ever experienced that.?