What can I do to gain weight???

I found out today that I’m 105 lbs😖 I haven’t been this small in 7 years😪 like obviously you know most would say stop stressing. Eat on a regular schedule. And in any other instance I would agree. However, even when I eat how I normally eat (which I eat almost as much as my dad and he EATS EATS😳) but even then I still can’t gain weight. Like I’ve always had problems gaining weight or even maintaining a certain weight. It’s always fluctuated frequently. The most I’ve ever weighed was 150 at 3 points in my life. The first time, my hips developed but I quickly dropped back down to 117 when I got sick🤦🏽‍♀️ the other 2 (and most recent) times, I was pregnant 😅. I had my son 4 1/2 months ago and I’ve lost 45lbs😣😖 and me being sick now doesn’t help either because my weight always goes down more when I’m sick. So what can I do? Like my doctor didn’t speak on it but I know I’m underweight for my height and age.