It seems to be related to low blood sugar, here's some quick web search blurbs that probably explain it: "Scientists were already aware that low blood sugar mi

It seems to be related to low blood sugar, here's some quick web search blurbs that probably explain it: "Scientists were already aware that low blood sugar might speed up heart rate by sympathoadrenal stimulation. In other words, low blood sugar may spark an epinephrine or adrenaline response in the body as part of our biological emergency alert system, that which induces “fight or flight” or in the case of diabetes, a strong urge to signal the body to “consume sugar now!” Coupled with low blood potassium, this response may lead to an irregular heart rate." "Blood sugar levels affect the heart when the levels are at extremes, either very high or very low. A higher level of blood sugar raises the blood pressure and heart rate. This leads to heart diseases. If you have low blood sugar levels you will experience a rapid heart rate and a drowsy, irritable weak feeling." That latter description far better explains how I was feeling. Anyways, I neither understood why I was feeling that way, nor it having any relation to my heart rate. Having my Fitbit on both gave me information that could help me identify what was going on, but also a way to know if it's happening again, so I can eat something quickly.