This sounds like a manifestation of anxiety imo. Some kids are so astute, they'll say everything is fine because they don't want to burden or disappoint the adults around them (my girls do this) not realizing that naturally, the issues just come out in other ways. Counseling has been a huge help for my oldest, I really think that's your best bet. I also found that reading picture books about managing anxiety has helped, since they provide ideas for coping mechanisms as well as an opportunity to discuss what's going on.