The "answer" that I'd offer is a question actually... Have you ever been examined for MS? What you describe sounds an awful lot like Multiple Sclerosis, but I may be somewhat biased. Since I have a fairly severe case of MS and was only diagnosed about a year and a half ago, I tend to "see" it everywhere.
Oh, I'm having really bad headaches, and very bright sunlight hurts my eyes...
Yep, that's MS.
Intermittently weak/spastic muscles...
Honestly, I effectively don't ever run, (well, except when chased) so I cannot speak to that, but my knees are CONSTANTLY on the verge of buckling when I'm walking or standing still, so as soon as I read your question I instantly thought: Yep, that's totally MS.
See your primary care physician, or go to urgent care, describe your issue (webmd[DOT]com/multiple-sclerosis/multiple-sclerosis-symptoms -is a decent resource for potential symptoms) and ask about getting an MRI.
As far as I know, that's the only way to diagnose (or rule out) MS.
I hope that this helps, and I very much hope that you don't have MS, I wouldn't wish it on my worst enemy... If you do, stay strong and try your best to stay positive. Some folks here, like Supermum, are a wealth of information, and even people that might be utterly full of crap do mean well.
All the best,