[Isabelle]

It is a known side effect, yes:
"The compounds coumarin and nicotinic acid found in fenugreek, for example, can have very potent effects on heart rate, blood pressure, blood sugar, and other bodily functions."
http://www.breastfeeding.com/reading_room/herbs.html

My guess is it could be an allergy to the nicotinic acid. If an antihistamin normalizes the heart rate, it's an allergy. Lowering doses helps too.