Obviously I'm not a doctor or anything, but I really do think that if you have a sinus infection it could just make you feel "off" and a bit foggy. I know when I get a cold I sometimes feel like I'm half asleep!
As far as your MS/ALS fears go, I recommend reading through some of the threads about it on this site. Many of them are very VERY reassuring. I went for an eye exam about a month ago and the optometrist said my optic discs looked swollen (which of course is often a sign of MS!!) and he asked if I ever felt dizzy/tingly/etc. and of course I hadn't up until that point... then for about a week afterwards i started experiencing every early MS sign in the book. I was convinced my vision was going and had pins and needles ALL OVER.
After I saw a couple more eye specialists and doctors who said I passed all the neurological and vision tests, and had my CT scan (clear), the symptoms started to go away and now they're all but gone. I had an MRI done a couple days ago and honestly I'm not even concerned about it, I'm convinced it will come back normal. My family doctor doesn't even think I should have had it at all because i'm showing no real symptoms but it had already been ordered.
My point is, MS symptoms are pretty vague but doctors can tell and if you get so tingly/numb/dizzy/out of it that you can't move, THEN you should start worrying. For now talk to your doctor if you're that concerned. But i'm almost positive they'll just tell you your mind is foggy because you have a sinus infection, as well as anxiety