No appetite ☹️

I am currently 19 weeks with identical twin girls. I have absolutely no appetite whatsoever! I have not gained any weight. The specialist I am seeing keeps telling me the importance of eating right and gaining the right amount of weight so the babies can get what they need. I’m trying so hard. I never crave anything and just now I had to choke down a sandwich just so I wouldn’t pass out from not eating. I don’t know what to do. If my Fitbit didn’t remind me to eat I would probably go all day and then end up passing out. I never had this issue with my other pregnancies. I have bad nausea and vomiting during the first 14 weeks but I haven’t since I have been taking B6 everyday. If anyone can offer any advice I would greatly appreciate it!