About a week ago, I started noticing that my heart beat would randomly speed up at times for a few minutes, causing me discomfort. Sometimes while I'm skateboarding, or sometimes just while I'm sitting down doing nothing!
I've also had two instances in the last week where I stood up and then got really dizzy for a few seconds (one of the times, I lost vision very briefly).
It sounds like your blood pressure is dropping when you stand up, which can cause dizziness and rapid heart rate both. This is usually caused by dehydration, especially if you typically run a low BP anyway. I would increase your fluid intake and if exercising heavily have a sports drink that contains some salt and potassium to get better hydrated. That should help.