Magnesium "tolerence"??? - What could be going on???

Because I was bothered by RLS (Restless Leg Syndrome) late evenings and when trying to sleep, I started taking a magnesium supplement (it supplies calcium too, to promote better absorption of the magnesium it says). And because I know my GI tract is somewhat delicate and all the stories about upset stomachs were scary, I obtained a "time release" formulation that releases slowly over several hours rather than all at once - hence no stomach upset. (Only a handful of time release formulations are available, so I don't have much control of the formulation  ...but that's not the problem, at least I dont think it is:-) Magnesium supplementation worked, the charley horses and involuntary movements and inability to stay asleep went away.

*BUT* after a few weeks of bliss, I find I'm needing increasing doses of the magnesium supplement just to stay even. And if I miss just one dose, the undesirable symptoms are as prominent as they were before any supplements at all. And even worse, I'm now suffering general muscle aches and pains that weren't there before, to such a level that it's sometimes hard to walk very far. (The experience is a bit like I imagine being "hooked" on some illegal drug must be:-)

What could be going on?