I go through the same thing. I'm not underweight but I do worry that I'm not eating enough on the days where my nausea is really bad. I do recommend the peppermint tummy drops of nausea though. I do find myself feeling more nausous after I eat and it's really bad at night. I would suggest eating small portions through out the day instead of alot in one sitting.
Due to being so little earlier in my pregnancy the weight gain is healthy, but I'm not sure about the over eating, ill have to ask next appointment, but nausea has made some days impossible to keep food down which could explain feeling extra hungry later on, but I don't know):
Has your doctor talked about having any concerns about it?