Hello and another scale is not moving question

I have been active on the site for a while now but had never properly introduced myself, so hello all!

I am 36 years old and was 149 when I started this weight loss journey again last month. My goal is to hit 130, although I would be pretty happy with 135. As a motivational tool to lose weight together, my husband and I entered a 3-month fitness/weight loss challenge at our local gym. The first month went great. Lost about 9 pounds and 4% body fat. I was halfway to my goal and super happy and motivated.

The scale hasn't moved a single pound in September. I watch what I eat, walk about 5-6 miles most days (don't log it here because I do it through my Garmin Vivofit) and lift about 3 times a week. I see a difference in my body but am bothered by the lack of movement on the scale.

This morning when I weighed in at the gym, the weight was still the same but at least I had lost 2% more body fat. The trainer that weighed me in basically berated me very loudly for complaining about the scale because he said I was not eating a "balanced" enough diet. This guy knows absolutely NOTHING about me and said this solely based on the fact that I mentioned I sometimes eat plain oatmeal for breakfast (his lack of professionalism belongs in another post!).

When I asked if the lack of weight loss could be due to the muscle that I am gaining, he said no, that if it was muscle both weight and body fat should be going down at the same time. My husband thinks that is BS. Is the trainer right?? I am so confused and still feeling a bit bummed not to see any movement on the scale!