Lump on Julii Cory

[Frodo]

Hello

One of my 4 Corys has developed a lump right below his mouth. I feel like it's size increases and reduces on its own. This is also my smallest Cory and I am always a bit concerned with him ever since I have bought it home. 
It is one on the left side in the photo. Tried my best with the picture. 
Thanks. 

Parameters: 

30G tank planted tank (swords, water sprite, dwarf hairgrass, frog bits)

PH- 7.0 to 7.2

Ammonia- Somewhere between 0 and 0.25ppm. I can’t really can’t make out for sure if it is 0.25 on the chart. I am regular with my water changes though. 
 

Nitrite- 0

Nitrate- Between 0 to 5ppm. 

[Cory]

Interesting, I think over time it'll probably rupture, and then you just need to keep it from getting infected and it'll probably heal up. So just keep an eye for now. 

[Frodo]

Update: He is still alive but he scares me sometimes by playing dead. He was lying on his back flat today. Thought he was dead, went to pick him up disheartened but he ran off! 
I can’t understand him. He looks healthy apart of the lump. It hasn’t changed in size. 
Is it normal for them to do this? 
 

Ph-7.2

Ammonia/Nitrite- 0

Nitrate- Between 5 to 10. 
planted tank. 30G 

[Frodo]

Update: The lump has cleared out on its own. He is now a fat little piggy. Couldn’t be happier. 
FYI- I didn’t treat him for anything since I didn’t know what it was.