80s RHR dropped to 60s within a few days? Cause for concern?

Female; 28; 105lbs; no meds or supplements I have suspected POTS, so my heart rate tends to run on the higher side. Especially my resting heart rate (often 110 when walking around the house and 140 when I'm in the kitchen). I started implementing a 10 minute walk twice a week last week. I haven't walked at all this week, but my heart rate is down by 20. It went from resting in the 80s two resting in the 60s, the lowest being 62-64, not too long after finishing my first walk. When I stand up, it's now reaching into the 90s instead of 120 / 110. I haven't changed my diet or anything. I've simply walked for 10 minutes twice. But now even after climbing stairs it's 80 instead of 130. That drop is weird to me. Should this be caused for concern? I feel pretty tired, but no other symptoms that would be concerning. I'm also 2 days out from ovulation so I'm wondering if this has to do with a hormonal change. Thanks in advance!