Taylor @Tee25 you may be asking here “which came first, the egg or the chicken?”. Or a sub-title for this Topic could be “Does body glucose level overreact during exercise?”. The answer to the glucose question and hence the Dexcom is YES/
A couple of things are happening here in addition to the possibility of Dexcom proving false readings - I’ll mention the Dexcom below. During the exercise you describe, I can believe that your BGL might very easily drop 100 mg/dl; I suspect at some point during your walk that the arrow was slanted down 45 degrees - maybe even 90 degrees. Your body has a natural intelligence that senses your activity - your mussels are crying for nourishment - and the t-cells scramble to your liver and demand that glucose be released to feed you. This glucose release is what is causing the post-exercise in your BGL. My “workaround” for the situation you describe - it may not be advisable for everyone. Most mornings I take a walk at 130 paces per minute for a distance ranging between 4 and 6.5 miles - a luxury of being retired. To avoid accidental dehydration and adverse low BGL, my water bottle has a scoop to Gatorade added - the real Gatorade developed by University of Florida athletic department loaded with sugar. I do NOT count these 20 grams of carb in any calculation and find my BGL at a very comfortable level up to 4.5 miles - for longer walks I rely on snack-crackers to feed my body enough so only very little glucose need to be released from my liver.
Possible Sensor malfunction: Depending on your sensor location, compression of insufficient hydration could cause the sensor to provide lower readings than actual. To operate accurately, the interstitial fluids into which the sensor is implanted need to be sufficient - keep yourself well hydrated. Sensor compression occurs for many reasons - not just rolling on it while sleeping. That very fine platinum wire is extremely sensitive - and can be compressed by movement of mussel, clothing especially if near a belt, etc. But I think your 90-100 mg/dl drop is actually just a natural effect of your healthy activity. I was uncertain, so to validate my fluctuation I carried along a BG Meter and checked - always comparing by BGM readings with CGM reading 15 minutes later.