Light and Dark frames - Indoor Pool

Hi,

I took my new sigma 70-200mm f2.8 and Nikon D600 to my niece's swim meet today and came away a bit perplexed...

I noticed when I had it in Ch mode taking rapid pictures, from one frame to the next the exposure seemed to be quite different...Could this be from the type of lighting there? I was sitting high up in the stands...same type of issue was apparent at different focal lengths...I shot auto ISO and manual...shutter was 1/640 and f was 2.8...

I shot earlier in the day at my sons swimming lessons at a different pool and didn't encounter this at all...there I just encountered my poor photography skills...LOL

Some images attached...

Thanks for the help in advance!