A friend did this to me years ago. We'd planned to go and watch a film on a particular day, it'd been about two weeks beforehand we planned it. Spoken/mentioned it in conversation many many times over the two weeks and then the night before I phoned them to double check where we were meeting, what time, should we get some dinner before etc etc and I was met with "What? Oh, I completely forgot? When did we plan?" which I couldn't even say anything because I was so confused.
So, anyway, they couldn't go because they had family visiting that day apparently. Which if they'd basically said "Sorry, family are round can we do it another time?" or whatever, it's the dancing around the truth I can't stand and the fake excuses and the fakest pretending to have forgotten I've ever seen in my life.
I was pissed for about half an hour and then got over it..... until later that night and I got a phone call asking if I wanted to meet him and a couple of others for a drink and without even registering that he'd cancelled with me or any of all that bullsh*t, he starts telling us all about this great film he'd watched today with his cousin. The same film we were going seeing.
I could've swung for him.
But, yeah, your story sounds similar. Someone who for some unknown stupid reason just can't be straight and needs to use the old "I'm not well" bullsh*t instead of just saying they have made other plans or whatever. I hate bullsh*tters.