Female has unidentified lump on side, please help

TLDR: Girl was swollen, treated for constipation, turned out to be part of the problem, but noticed large, asymmetrical lump on her side. Looking for identification.

About a month ago, my female Sini became egg-heavy with all the other girls in the sorority.
However, after everyone else had either dispelled or reabsorbed their eggs and gone back to normal, she stayed bloated. She's quite a few months older than most of the other girls so I gave her a little while longer, but when two weeks ago she was still bloated and had become even bigger, I QT'd her and treated her with Epsom salts for 3 days, hoping this would help with the swelling. She passed a very large amount of Feces the first day, which made me hopeful it was only constipation, but though her swelling went down, it was still very present.

Having no other change, I put her back in to sorority. Her swelling has stayed the same, but I hadn't really looked at her from above until the water change today, when I took out all my girls so I could move around the decorations around without hurting anyone. It was then that I could look at her from above, and noticed the large protrusion on her left side.
I've decided to put her in QT until I know just what it most likely is. She has had no change in behavior this whole time (except when I've separated her from the other girls, she tends to pout ^^), she retains a healthy appetite, has no trouble swimming or eating, and has maintained her position in the hierarchy.
I have a feeling it's a tumor, and if that's what is decided, I'm just going to put her back with the rest of the girls and only do anything special if she has trouble eating/swimming. It'll be sad, but I've given her a pretty darn good life and I'll just make sure the rest of it is just as happy :]
As it is just her, over several weeks of water changes, I'm fairly certain it is not water-related. The sorority is a filtered 10 gallon, heated to 79-82*F, with an average PH of 7, and the water is a little on the hard side. It's not technically planted, but there is some driftwood, java moss, and dwarf water lettuce.

Attached are some photo's of her from above. You can see how the lump has caused her side to protrude almost to a point. Any thoughts, suggestions, or questions are welcome and appreciated!

Housing
What size is your tank? 10 gallon
What temperature is your tank? 79-82*F
Does your tank have a filter? Yes
Does your tank have an air stone or other type of aeration? Bubble Curtain at back of tank
Is your tank heated? Yup
What tank mates does your betta fish live with? 5 other girls, an apple snail, and a juvenile albino bristlenose pleco.

Food
What type of food do you feed your betta fish? Hikari Bio-gold pellets every day, OmegaOne Betta Buffet flakes every other day, Frozen Brine shrimp once a week.
How often do you feed your betta fish? Once to twice a day.

Maintenance
How often do you perform a water change? 30% every third day, 50% every third change.
What percentage of the water do you change when you perform a water change? (See above)
What type of additives do you add to the water when you perform a water change? Jungle Start Right water conditioner, API plant food.

Water Parameters:
Have you tested your water? If so, what are the following parameters?

Ammonia: safe
Nitrite: safe
Nitrate: safe
pH: 7
Hardness: medium hard
Alkalinity: okay

Symptoms and Treatment
How has your betta fish's appearance changed? (see pictures)
How has your betta fish's behavior changed? Not at all
When did you start noticing the symptoms? A month ago in general, a week ago that her swelling was asymmetrical, today the actual shape of the lump.
Have you started treating your fish? If so, how? QT with warm, clean water for closer observation.
Does your fish have any history of being ill? Nope
How old is your fish (approximately)? A year and a half-ish? Definitely old enough for more genetic/long term issues.

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