Heart rate spikes

Hello,

I am 36 and work out 5 to 6 times a week. When I am swmming my heart rate will get to around 150 to 160. Sometines thou out of no where it will spike to 200 to 220. I always stop swimming when it does this. If I hold my breath and bare down(like going to the bathroom), I will feel 1 big heart beat and it goes back down to a normal range. My chest will feel a little tight and I feel like I have a limp in my thoart. After a few minutes thou everything is back to normal.

What could be casuing this?