The response from your group members could be a bit of jealousy- which often happens among women who aren't comfy with themselves (regardless of size). Or, they could be expressing legitimate concern with illegitimate words. On one hand Americans are accustomed to seeing each other at larger weights, but we're accustomed to seeing stick figures on the runways- many of whom do have eating disorders. I'm 5'4" (an inch taller) and used to weigh 113 too- now I'm 121. At that time my BMI was on the edge of healthy. IMO as long as you're maintaining weight the healthy way, getting all your nutrients, & not over-exercising - and it sounds like you're doing ok in those areas - I wouldn't give too much weight (as it were) to their comments.
I'm with Wylie in that I agree that a support person shouldn't say certain things. I recall one of my friends, who also had a thin build, in OA years ago expressed concern that I was losing too fast. Not once did she tell me my loss was "scary," but she simply mentioned that I might bring it up to my sponsor. I never took offense to that approach.
Maybe you should evaluate what the WW group brings to you at this point, possibly look into replacing it with a different support system. Obviously WW has helped you do great things, but it could be the time to supplement it with a more positive resource.