Endomondo/Strava +iPhone6 +altimeter
GPS may be accurate +/- 15 meteres.
http://gpsinformation.net/main/altitude.htm

If you have a GPS device with barometric measurement - then this can improve.

Should you be worried? If you're using it for cycling then probably not - it is only an indicative figure of metres gained or lost.

If you're trying to land a plane - then yes. :)