Is this dropsy or is this guppy a pregnant female?

I've had this tank of 7 male guppies since March. They are doing really well. Today I noticed that the black guppy has a huge abdomen. He is acting completely normal. I look at them every day and have not noticed him getting larger. Is there something wrong with him, or is he actually a she? I've never seen the others chasing him. I took a couple pictures with the other guppies because he had been thin like them until now. I have not added any fish or changed anything in the tank.

9gal Fluval Flex. I change 50% of the water and rinse the filter once a week faithfully. There are a few live plants, driftwood, petrified wood, and a large air stone. These have been in the tank since before the guppies. Ammonia 0; Nitrite 0; Nitrate 20
I appreciate opinions. Thank you!!

The black guppy is a male. If it got fat overnight then it's fluid build up in the abdominal cavity. This is normally dropsy but the position of the lump is not in the normal position for dropsy. Dropsy normally stops the fish from eating and they die within a few days of developing symptoms (getting fat).

It could be a cyst or the fish has eaten a lot. There's no treatment for cysts so it's a matter of monitor and see what happens over the next few days to a week. If it goes down in the next 24 hours then it's probably food. If it stays like that for a week then it's probably a cyst.