Unbelievable Weakness - Searching for Cause

Has anyone else experienced incredible, all-over weakness that makes you feel totally listless, whether or not you are sitting? Can active Crohn's by itself be the cause or is there some overriding additional factor? Just had blood work, will wait a while to get results. Really feels overwhelming, so I'm skeptical of any "marginal" issues that cause a bit of fatigue/weakness.  Any theories would be helpful, a few facts below.

Clues:

Normal B-12 levels last checked (although time before that was in 200 range).

Taking Entecort and Humira - can these have this side reaction?

BUN, sodieum, potassium - OK

Pulse beats higher when standing up - endocrinologist said i may be dehydrated, but the weakness is severe so doubt it is this.

Severe insomnia, sometimes take Ambien 

Iron/ferratin levels?

My pulse also beats higher when I get up, I don't think that's too unusual. What heart rate's are we talking about?

Any symptoms of MS, or just weakness all over?

Does this weakness persist over hours or days, or is it something fleeting that comes and goes?

For what it's worth, during a few of my longer "flares" that lasted several weeks, I've had days where I was so weak I could barely take a shower, or barely even sit upright for very long, yet I wasn't dehydrated, wasn't malnourished, had no diarrhea, and my blood work showed nothing unusual. The best way I could describe it is I felt like I had the flu but without the high fever. In fact I would say that is one of my primary indicators of flaring for me, since I usually don't have the big d even in flares.

So, yes, it's possible that Crohn's itself is causing your weakness. At the same time, it's always wise to look for other, or additional causes, such as you're doing.

Low potassium makes one feel weak like that. I'd get all the checking done. I know I felt that way on Remicade because it was not for me. Be interested to hear what you find out. Hope you feel better soon.