There is always 2 sides to every story. Some of the guys I know in my world, that after they blaze, their worthless!! They get all lazy, dont want do anything except play games. IF, I was a girl, and was around that everyday, well phuck, I'd want a change also.

Talk honestly and up front to her to find out her exact reason to dump the relationship. Maybe it's not the weed, but the side effects.
If she will not elobrate, then maybe, it's time find other ladies in your comfort zone.

I'd talk or walk.