Losing Fitness Over Past 2 Months Despite Training Consistently

I started running again in June this year and used a Garmin Run Coach training plan for a 10k that completed at the end of July and then another one for a half marathon that completed a couple weeks ago.

I’ve made a lot of progress since June but since about late October my Garmin VO2max has been declining. Went from 56 to 55. I recognize this is an imperfect estimate from Garmin and I don’t care about the absolute value of the number but the trend is concerning to me.

My overnight HRV during this time has plummeted from an average ~55ms to ~45ms. I don’t drink, don’t smoke, and have been running ~25-35 miles per week during this time.

I feel mostly ok health wise. I definitely think I’ve been getting worse sleep during these last two months but haven’t changed any of my habits/diet. I just don’t have any clue what to do anymore or what the cause could be. It’s extremely discouraging and having trouble keeping motivated. Why train if my fitness just keeps decreasing?

What really sent me over was today’s run. 1hr base run struggling to stay below 150bpm @ 8:30 pace on a cold day (avgHR ended up being 146bpm). Compared to a couple months ago I ran 2hrs @ 8:30 pace and easily stayed below 150bpm the whole time (avgHR ended up being 141bpm). Similar temps and elevation gain.

Has anyone dealt with anything like this or have any clue as to what’s going on?