IMHO the important point to grasp is that stalling (in either the positive or negative sense) has no relation to 'G' as in gravity. It is entirely related to the angle of attack of the wing, which is (roughly) down to a combination of the acceleration on the airframe (producing loading of the wing), and the speed at which the air is passing the wing.
In 'conventional' flying, (g)ravity is a handy way of providing that acceleration so as to allow non-dramatic stalling - we simply slow the airflow until gravity provides enough acceleration for a stall.
More dynamically, as has been said, you can generate that stall in either direction in any attitude with sufficient provocation. The avalanche is a positive manouver, and (should) start about 45deg before the fully inverted position - and yes, flicks are a (fairly) violent experience.
If the stall warner was going off, it was a positive stall. Stick position (generally), and whether you're being shoved into, or out of the seat are good clues
