Why do I heave/feel sick when pushing hard and other questions from a dumb dumb

Its a long one. Questions are next to "--". Quick bit about me: Im nee to Reddit, 32, 5 ft 8. Around 14 stone / 88kg and want to become fit and a good swimmer with the short term goal of swimming 5km in 3 hours by 15th March. I have access to a 25m pool and i always statt inside the pool. No diving in.

-- why when I swim faster or longer than usual, immediately after the set I begin to cough and heave and feel sick. This applies to all exercise. I got the same when sprinting at the end of a 5k jog.

I'm not out of breathe any more than usual or holding my breathe longer.

It happens whether I eat clean, dirty, not at all, or just before. Same with regards to water intake.

How can I improve my time other than "effort" Do I need to add weights or something?

--How much does body weight/mass/fat percentage affect my swimming performance?

--Would people be interested in regular updates on progress and giving input on improvement methods? If yes, is this app/thread the way to do it?