Can’t gain weight??

Throughout my first trimester I lost about 10 pounds, I’m not overweight by any means so it’s it’s all weight that I definitely need. Well now I’m 21 weeks and still have not gained back my 10 pounds I lost. I’m about 3 pounds off but I still feel like I should have gained more by now. I weigh myself regularly and yesterday lost another pound? I feel like all I do is eat and lay on the couch, plus I see my belly growing and changing and would definitely think i put on a few pounds. I’m really not understanding how I’m not gaining any weight. I have not expressed my concerns to my doctor but they weigh me at my appointments and have never said anything. I eat 3 meals a day and snack occasionally throughout the day. Has anyone experienced this before? What was the outcome? How did you manage?