How can I gain weight even though I’ve lost a lot of weight practicing martial arts?

I’m not sure if this is a question that fits in this sub, but I need help. I’m a guy who loses weight very easily. the mere thought of losing weight is enough for me to lose weight. I’ve been doing Muay Thai for about two months and I’ve lost 30 pounds in that time, going from 170 lbs to 140 lbs, and I look like a corpse. The most annoying thing is that I feel like I’m not progressing, because even if I develop a good technique, I don’t have the strength to execute it. What should I eat or what should I do to gain weight?