It's possible the soy sauce has monosodium glutamate (MSG) in it, which for some people (myself included) causes palpitations. Also, there is a high sodium content in soy sauce. Additionally, if you've been using it on top of other foods that have MSG (like Chinese food from a restaurant), you might be getting a double whammy, so to speak.
I would lay off of it and if that doesn't solve the problem, call your doctor and get in for an evaluation.
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