1. You were distilling an azeotropic mix of DCM+Methanol(+ godknowswhat).
2. You were initially distilling at such a slow rate, that the small amount of DCM condensated on the bottom of your stillhead thermometer, and while evaporating back into the rest of the stillhead, took away so much energy from your mercury in the thermometer, that you got a wrong, lower reading of the DCM vapours, passing too slowly by the thermometer.
3. A combo of 1+2. LT/
PS: washing your raw DCM mix with clean water first, then separating (in a separator funnel) from the mix that water which will have taken up any possible amounts of methanol and other water soluble polutants, will spare you from nr 1. and 3.
DCM will not mix with water.