That's awesome. Yeah, it sucks training when you're not feeling well. That was me for a weak or so. I think I was losing weight too fast, which was a good portion of water weight, and it really threw off my electrolytes. My heart was beating funky, and I was feeling more tired than I typically do. Kind of scary.
Anyways, I did a little research and found a video with a guy explaining that the same thing happened to him, and his Dr. told him to get some Magnesium supplements. Once he did, it went away. Apparently, Magnesium is the key mineral that regulates the valves in your heart, so if you're low, the heart valves can beat faster, which is what was happening to me.
I have some Mg pills (but hadn't been taking them), so I took a good dose of them at two different times in the day, and my heart rate went back to normal very quickly. Now I need to get back into it, especially with that nugget of information available to me now.
Cliffs: If you're going low carb and/or losing weight quickly, and your heart is beating oddly, it may be a shortage of Magnesium.