Inadequate insufficient weight gain

Hello. Usually I don't use nurture for the discussion, but i really need some advice and support.

Thursday I went to the OB for my routine 28 well visit, and my doctor informed me he was displeased with the amount of weight that I had gained, that such a thing could cause a problem.(at this point I've gained 8 lbs, was of normal weight, 26 BMI pre pregnancy). Mind you, I have two children, with whom i gained between 40 and 60lbs with, but my last full term pregnancy was almost eight years ago. We went into the ultrasound room, and they measured baby. His body was correct length, and head measured 30w5d (which was the gestational age advertising to my period, but they switched it to two weeks back.) Baby weighed 2lbs, 15oz at the almost 29 week visit. They cleared me to go home, but told me if I don't gain more weight I will be put on a regimen, and that I was harming my baby by now doing so. I'm having a hard time with this, because I eat as healthy as my cravings allow, and always eat when I'm hungry. I'm not even a smoker! Anyone have any words of wisdom?