Really scary

I'm already a small person but some how I keep loosing weight I eat constantly all day. I just don't get it. My doctor told me to eat more and try and maintain. I'm coming into my second trimester and it's just annoying so now I'm force feeding myself trying to make sure I get enough calories..... I'm eating like 4000 a day....... Ugh..... I want too be like normal people. I'm so worried