Breastfeeding Issues

I've taken my daughter to the Dr twice now and both times they tell me that she's not gaining weight the way they want her to they tell me to feed her more change my diet because I was on a strict diet trying to lose weight now I've got to eat whatever to try to help her gain weight I've done all they ask with no luck I've been crying my eyes out I've never had this problem before my husband is super supportive and telling me it will be OK but I feel so bad that me wanting to get healthy may have hurt my daughter any thoughts