Likely not, more probable is a change of the atmospheric pressure, or possibly a wind pressure (happens at fast bike rides too). But it is hard to tell when we do not know the true elevation profile. You may want to go to the web interface of Garmin Connect, and enable the Elevation Correction on the page of the concerned activity. It will then help you to compare te topographic elevation with the recorded one.
Do u know a way i can check if my barometer is working
I'd tell that the easiest would be looking at the barometer screen and watching whether it changes with barometric pressure. Or you can look at the altimeter screen and climb a floor or two with it, to see whether it reacts.You can also make a test dive (or a freedive) to a known depth, or with another gauge.
The MK1 has constant and ongoing issues with the elevation. Many posts before this one describe various issues with various activities. The elevation measurement is broken, period.
The issue was potentially fixed on the related Fenix 5 but there is no firmware update for MK1 for long long time. MK1 users are just an abandoned user group...
It is worthwhile reading this:
to understand the issues and difficulties with measuring altitude on a watch.
That said, if the weather (read: pressure) changes, the altitude reading will change as well, even if you don't change position. This is why watches should always be calibrated before starting a trip, and throughout your trip, anytime you have a reliable reference point. If you encounter weather, continue to calibrate the altimeter manually at known sites (using either a map or elevation markers on the trail). If you travel to a different part of the world, make sure you calibrate the sea level air pressure reference to that area, as it changes based on where you are.
As Marcin said, the elevation measurement (or calculation) is broken, period. My MK1 was replaced, GarminHeath supported to check the barometer itself is ok, they has asked for log files, but nothing at all happened on the Garmin side. I finally ditched my MK1 and changed to a Fenix6X for the last winter season because I was just sick and tired of this Altitude issue and Garmin ignoring this problem.