That sounds like me. I can be wracked with guilt over the smallest things.

What I would say is that you didn't open the door, fine. That's in the past now. You can't change the past. If he knocks again, you can choose to answer. If not, it's your choice what to do. But don't blame yourself for not wanting to answer the door to someone that hasn't been friendly to you when you haven't showered in a while. Even if you were feeling well you could have chosen not to open the door.

And hypothetically, if it was something really bad, and he had to tell you specifically, he could have slid a letter under your door. It could be he hadn't seen you in a while and was wondering if you were still kicking.