Hi, I'll do what I can to help, but will be honest... the photos aren't real clear and there isn't much information provided.
My first question is: How big is this fish?
Then I also need to know what the water params are for ammonia, nitrite, nitrate, and pH? If there is medicating that needs to be done I will need the water params in order to find a safe treatment plan for you.
And can you tell me the water temp? (this is also important in deciding medications)
It's hard to tell in the photos if this is a bubble or a solid mass. There are a lot of possibilities for what this could be, which makes it difficult to diagnose something like this via internet. The appearance of the red veining is evidence of a bacterial infection, but the cause of that infection is not so obvious. The infection would be secondary based on your description and the photos.
When the fish is passing feces, can you watch to see if there is any sign of little (very tiny) red worms protruding from the anal opening?
It's going to be very important to get as accurate of a diagnosis as possible before we choose medications.
As much as I would like to offer you hope, at this point all I can say is to get the info needed to me asap and we will take it 1 step at a time. I make no promises, this is a very advanced situation... but I will do all I can for you.