Bode plot starting from 180?
If you have a non-inverting amplifier, then you are interested in the phase shift away from 0° as the frequency changes. If you have an inverting amplifier, then you are interested in the phase shift away from 180° as the frequency changes.

does it mean I have poles at the origin or it's very stable?

No, it doesn't. An NMOS common source amplifier is an inverting amplifier (see above).