Extreme Weakness from MSM

I started taking MSM about a week ago and at first had a significant energy boost from it.
But now it just really fatigues me to the point getting up a flight of stairs is a challenge. I'm just too weak to do it easily.
Anybody got any ideas as to why it's doing this? I read that it's antagonistic to calcium so could that be why?
Any help is much appreciated.
Thanks,
Ben.
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