Hello, I'd Like To Know Why I Am Having A

Hello,
I'd like to know why I am having a metallic-y/tinny taste in my mouth after have certain back (for lack of better term) electronic massages to my middle- and upper-back. I have had these massages 2x, and both times I got the tinny, metallicy taste which proceeded to last a few days.

The treatment (every 2 or 3 days) involves 4 rubber cups placed on my back that pulse and send energy into my back at different paces and rhythms. Then, a Brookstone-type "horse collar'-type pad is placed on my shoulders which proceeds to tap and thump on my upper back, also at different speeds and rhythms. The treatment level is noticeably intense; at points my back, inner gut (along the front/ventral side of my torso), and upper ab muscles will start to rhythmically spasm with the pulsing of the rubber cups on my back, but is never never painful or discomforting.

I am also getting therapy for other body areas and none of these have caused this tinny/metallicy taste in my mouth.

I briefly mentioned this after the first such upper back treatment, but was told it didn't seem a major issue; today was the second time and am immediately having the bad taste again. (I have been eating regularly, no medications of any kind; can't think of anything to confound this correlation with the upper back therapy).
The only other times I have had a similar experience is when I eat "low-carbo" for at least a day or so, also whenever I go at least half a day without eating anything at all.

What could possibly be the matter?

Thank you very much for any input you may offer.