Gaining weight in deficit!?

my calorie deficit was working so well for me. I lost 25 pounds doing it! I’m 5’9 and a half and currently weigh 147. A couple weeks ago I was 145! I’d like to get to 140. I’ve been eating at 1660 deficit and it was working and then stopped. So I decided to move into maintenance cals which is like 1800 ish. And im hoping when I go back into my deficit it’ll start working again. Idk how it’s possible to gain weight when im eating under what I burn!! Help :(