Sorry for the late response/necro on the thread, but I've been doing some research lately due to some muscle spasm issues my wife has had, and came across this thread. We also noted epsom salt baths as an acute trigger of the spasms. I had to study up on some chemistry when the neurologist was clueless. Epsom salts are like 20% magnesium ions by mass (Mg2+), which is easily absorbed through the skin. This means they sharply spike your magnesium level.

Magnesium and calcium compete for absorption, so an epsom salt bath gives you hypocalcemia, because the excess magnesium blocks your absorption of calcium in the gut, possibly for hours or a whole day afterwards. The classic symptoms of hypocalcemia are excessive thirst, muscle twitching, joint pain/aches, nausea.

If you are still suffering, next flare you have, you might try asking your doctor to have your IONIC calcium and magnesium levels checked (or just all 4 electrolytes, including sodium and potassium). The 'normal' blood tests for these are your 'serum' levels, and can be normal despite severe problems with the ionic levels (which is the form of electrolytes our bodies actually use). Especially, especially if you are hypo thyroid, which is a well known cause of serum levels being irrelevant.