My 8 year old grandson was running in the swimming pool & told me his heart hurt.  I felt his chest & his heart was pounding really hard.  I had him sit down for a little while & he said it was better & then said it hurt a little, but started hurting a little in the ribs & then the tummy.
Hi...if it is unaccustomed swimming or exercise then it is quite natural and normal phenomenon. But if it is a regular who is getting this suddenly then it needs evaluation. Hope you understand what I am conveying. It depends on whether it is a recurrent thing or a single episode too. Regards -