My smile and demeanor projected a sense of calm to everyone but me.
Did something happen to you that made you feel that way?
No, I'm pretty sure I just developed issues
If you're feeling anxious inside, you should let someone know about it.  Hiding your feelings from others will prevent you from getting the help that you need.
Don't tell me how I should feel or act. I need to keep up my happy face.
I understand that you are going through a lot. If you have to keep up appearances, you should do what you need to do but you shouldn't try to hide your anxiety.