There are a couple of factors to consider here.....you were tired, and didn't feel a 100%, and that's all you needed to tell your friend. We all have days like this, and it's best to just be honest. This way your friend won't think it's anything to do with him. Don't go through life worrying about what others think of you, what YOU think of you is all that matters. I think everyone does this at times but we can only be who we are and people will accept us, and those who don't probably wouldn't have anything in common with us anyway....and that's okay. Be yourself and allow people to get to know you for who you are. Just don't beat yourself up over times like this, a quick call, text, whatever to your friend to just say hi and telling them to have a good day would be nice at this point, this gives you the opportunity to apologize for being so quiet and to explain why you were...keep it short and sign off in a perky way. Hope this helps!
Afraid I can't offer any diagnostic advice, but this happens to me too.
Councellors have told me it's dissociation before and a symptom of a mood disorder, but that is a personal diagnosis in consideration of my individual context and there are many causes of these feelings - your fatigue alone could have caused it, for instance, or your illness.
I find the best way to handle it is to just go with it - if the person you're with wonders about it, just tell them you're feeling quiet or blank. It's often quite a peaceful state for me, if not particularly useful, provided I don't let myself get worked up about not having any thoughts.
Good luck!