bloated guppy

Please can someone help, my guppy is around three months old. He was born to one of my female guppies so I had him since birth. These last few days I noticed he was staying by himself and not interacting with the other fish. I also noticed he has a swollen belly so I wondered if he was...

After eating lunch today my guppy suddenly got a fat belly and is floating pointing nose down. She acts fine otherwise and is still showing interest in picking at algae. Any ideas?