I'll let Mayo do the diagnosing, but some of the recent symptoms you mentioned, along with the weakness, made me think autonomic nervous system issues. Of course, nausea, light-headedness, and tinnitus can be any number of things, but you might want to google POTS (Postural Orthostatic Tachycardia Syndrome) as well if you're having issues with low blood pressure and heart rate increases when you stand. I don't know enough to know, but maybe? POTS symptoms would extend way beyond riding, though.
And let me add that the above is completely inappropriate internet diagnosing on my part....
And let me add that the above is completely inappropriate internet diagnosing on my part....