The heart rate going up is a normal response to fasting I would imagine. The cirulation system is vital of course to the fasting system. It is important both in the breaking down of fats and other stores and 'digesting' them through ketones and also in the release of toxins which have also been stored by the body while not fasting and unable to expell them. It is likely that the increased heart rate is just due to the fact that the circulation system is the focus of the fast at that particular time. At other times it could be the liver, or the bowels, the body follows a list of 'repairs' in the order necessary to heal the body most quickly and efficiently.
The release of old toxins back into the bloodstream can also cause similar effects to the ones they had when you first ingested them. Caffeine is a perfect example. It could well be a combination of the caffeine and the increased need for toxins to be released through the blood that is causing your heart to work harder. Toxins can also cause worse symptoms such as headaches etc if you are susceptible to them, they will all pass once the toxins have passed out of your system through the bowels, sweat glands or even the lungs.
So it sounds like you're doing fine. Good luck continuing. :)