I'm quite a quiet person but I make a lot of effort to socialise at work and be warm and friendly. However, every now and again a colleague will say "oh you seem tired, has it been a long week?" or "you seem stressed, are you ok?" and things like that when I feel perfectly fine and content. It always takes me by surprise as I never notice my behaviour/attitude has changed. I end up feeling really embarrassed and saying "oh yeah I'm feeling kinda tired" or making up that I have a headache or a reason for my supposedly different behaviour.
What might be causing this and how can I address it?