Hi there,
It is very scary when our heart rates does weird things. What you describe could be exercise induced atrial fibrillation or just innocent PVC's and PAC's which are just irregular heart beats, but not dangerous.
You should definitely go and see your GP and get some blood tests done. If you have low levels of potassium or magnesium, you could get these weird rhythms. Your GP can also then decide of you need to see an Cardiologist.
Take care and let us know how you are doing.