I took Fenugreek for a few months. No side effects. Then I stopped for a month or so when I ran out. Started it again and got dizzy spells. It took me a while to put 2 and 2 together, but when I stopped it again the dizzy spells went away. My milk supply stayed the same. Often it boosts and maintains your supply after stopping, sometimes it goes down.
I know that's not exactly the same thing as your symptoms, but yes I've also read it can cause low blood pressure AND low blood glucose. Just because it's herbal doesn't mean it has no potential systemic side effects. Personally, I'd probably carefully weigh it up, as heart palpitations sound a bit scary. A check of BP etc with your family doctor couldn't hurt either.
Have you tried eating oatmeal, drinking lots of water, pumping at the ends of feeds, etc to keep up supply?