I think you should visit your doctor. S/he may want to do a test such as a holster monitor to see what your heart is up to.
I have a health condition (POTS) where my heart beats too fast mostly in response to posture change, but also other factors such as dehydration. I have low blood volume, so I have to consume a ton of fluid. The thing about this is, if I'm not careful, I can wash out too many electrolytes and I start getting palpitations like you're describing. I have learned electrolytes use one another balance each other out and do their thing, so you want to get plenty of all of them. You can easily get these levels checked by your doctor.
If you're interesting in learning how to up your intake of electrolytes (sodium, potassium, and magnesium) here's more info...
Sodium is pretty easy to take care of.
There are primal friendly foods high in potassium such as spinach and Swiss chard. A cup of either has about 25% of your daily requirement. Scroll down to the chart on this page for more food ideas: [url]http://www.whfoods.com/genpage.php?dbid=90&tname=nutrient[/url]
Nature always seems to know what it is doing... The cup Swiss chard or spinach also contains 35-40% of your daily magnesium. I know this isn't primal, but I love my Epsom salt baths! Here's a page with info on magnesium: [url]http://www.whfoods.com/genpage.php?tname=nutrient&dbid=75[/url]