Did this happen at the same time last year if so it may be seasonal alopecia . Roge gets this, two years ago it was just on one side I had him with the vet who put him under to do a skin biopsy which came back clear and also a thyroid checked which came back normal so seasonal alopecia was the only thing that we could come up with, Last year it was much worse he lost a large patch on each side, I started giving him evening primrose oil and the hair grew back in no time however I dont know if it WAS the oil or just a natural regrowth. The last two years he started to loose his hair about the end of febuary so this year I have been giving him the oil since the begining of january its all trial and error at the moment with us only time will tell.
With your girl loosing hair and weight gain I would be leaning towards thyroid.
No it happened around september