Neon Tetra With Funny Lump

After doing a WC tonight, and then watching my fish swim around, i've noticed that one of the neon tetra's has developed a lump underneath. As i've not seen a neon ready to drop eggs before, and am new to fish keeping, is this just nature or something i should be worried about.

a few of my cardinals have had this before, it seems to come and go.... they all seem healthy and well though . ill be interested if its a bad thing. it seems to be when the fish are smaller from what ive noticed.

had you fed them recently - looks like a full stomach to me.

the neon is eating well, swimming around but not as active as the others. i'll just keep an eye on it i think to see what happens. but if anyone has any idea if it something to be worried about please! last time they were fed was 13hrs ago

Could be eggs, but in my experience you'll get more than one heavy with eggs at the same time. Could be a pot belly - my rummy noses get that after feeding, but it's usually gone within 3-4 hours, certainly after 13 hours it's gone. Could just be a constipated? In which case you could try not feeding for a couple of days and then feed them some deshelled pea bits. If it's not any of those things then I've no idea!