The pulse mystery

Up until last winter, my average heart rate during an easy run was between 155-165. Then, in February, something strange happened. Within the range of a few weeks, my average pulse rose and rose until it settled at 170-180. At first I thought it might be a lingering infection. But when my pulse didn't drop back to its usual levels as the weeks went by, I was completely baffled.

There was no logic to it. Speed didn't seem to matter, in fact my pulse was a bit lower during some of my faster runs. Ditto with time of day, route, hours of sleep or how tired I felt. There was no connection to the perceived exertion; some days when I felt I worked hard, my pulse was closer to 165. Other days when it felt easy, my pulse was closer to 185.

Today, when I ran home from work, my heart rate averaged at 146. I was running faster than usual, and it felt tough at times. So, who can solve this mystery? Please don't tell me I need to run faster...