Try running htop in one terminal on your VM. You should get something that looks like this
It will tell you what is using memory and how much. Maybe try running
classify-sklearn in a seperate terminal will you have htop open and look at how much memory is used.
EDIT: You'll probably want to sort processes by memory usage in htop, you can do this by pressing F6, then using the arrow keys to select "PERCENT_MEM" then pressing enter.