Neon Tetra bloated

Hi everyone

I bought 7 tetras 2 days ago and all 7 are still alive, the only problem is that one of them is really fat/bloated this morning in his belly region. He doesn't appear to be sick and is still brightly coloured, just fat... This is making him swim lobsided with little bursts of movement. I don't think he or she is old enough to lay eggs and I'm not really sure if and when they lay eggs they look like pregnant guppies.

Can anyone please help, I'm really worried

PS. We just did a water change just for incase