i agree with "tat", there's no reason for you to lose weight unless u want to. it's your body! in relationships i feel we need to love the inner person, not the outer "shell", justme.
i find his recent actions perplexing that he can be swayed so easily to follow the other gal's opinion so readily. and what business is it of hers to to nag you about your weight? so does he really have a "backbone"? is this quality in him something you missed before? if so, i'd be wary. know you'd like to keep the relationship but is this just the tip of the iceberg of his true personality?
imho, you are not overreacting at all. i'd be very uncomfortable if i experienced what is going on. there's a thing called a "triangle" in psychology. 2-3 ppl talking about the other 3rd or 4th person in a negative way behind the targeted person's back. it is considered very unhealthy.hope you can follow what i just referenced.
hugs. let us know how this all works out. we care.