Calling the pedi is a good idea. My nephew had that and he wore one of those heart monitors around. He might have been about 5. Turns out he was actually having anixety/panic attacks, but could not explain and no one could figure out the cause of the panic attacks. Not saying that is this, but your description is what he gave... he is almost 12 now and still has anxiety issues but is very well controlled. Good luck.
Phoebe Grace 6-22-11; 8# 1oz, 21" long/19# 12oz and 30.5" long at 1 year; 23# and 33.5" at 18 months! And oh my! Grew 3" in 3 months! 36.5" at 21 months... long and lean like her mamma was growing up.
Me & Geoff, 39; DD Phoebe, our June Bug