Welcome to NeuroTalk.

I'd like to offer up two early suggestions:

1) you say she had B testing? Was this B12? If the doctor said
"normal" to you, I'd suggest you get the actual test results. U.S. lab ranges go down very low to the 200's...and this is no longer considered "normal" with the new medical information out there.
If your daughter is down there, she could do with B12 treatment.

2) Low potassium could be a culprit too. The RDA for potassium has just been raised to 4.5 grams a day. Many people do not get this thru their diet. And weakness is one sign. There are also genetic conditions, that affect potassium movement into the muscle. One is called familial periodic paralysis. It is hard to catch with testing because the blood levels of potassium tend to change rapidly after a weakness attack.
This website will give potassium content of many common foods:
www.nutritiondata.com
If you check her diet, carefully, you can see how much she gets daily.

And also, make sure she is not "purging"...young girls do this to lose weight, and keep it a secret. When this is done, with laxatives, potassium is lost in the loose stools and the body gets low that way.

We have an MG forum, and they are very helpful over there...so I suggest you visit them when you get a chance.

http://neurotalk.psychcentral.com/forum77.html