Hello and welcome to FF. Don't think a tumor due to frog listing to side with bulge; which indicates its not solid, but has gas. From description and if condition is stable last couple month does not sound like bloating. Seizures can be related to bulge if it's displacing internal organs against the frog spine.
Have you tried to carefully palpate bulge? Maybe can do so and describe what it feels like and if it displaces or is hard/soft? Do you have a veterinary with amphib experience locally? An x-ray and surgical draining might be required.
Sorry if can't be of more help; but if condition is chronic and stable it's kind of hard to figure out. Please do take a look at this questions and see if anything here points to a possible problem. Also, has a recent test of your well water shown any heavy metals? Good luck!
“Trouble in the Frog Enclosure”
The following information will be very helpful if provided when requesting assistance with either your frog or enclosure. To help with your questions, please utilize the below list and post the information in the proper forum area to get advice from FF members that keep the same frog. This will allow for little confusion and a faster more informed response.
1. Tank Size and volume of water in it (full, 1/2 filled, etc.)?
2. Number of inhabitants including all frogs and any fish?
3. Water source and any conditioner treatments?
4. Water Temperature and how is it heated (if so)?
5. Water chemistry levels: pH; Ammonia (NH3); Nitrites (NO2); Nitrates (NO3)? Note that if you do not have test kits, most aquariums/pet shops will do complimentary tests if you take a clean water sample.
6. Describe any filter system including model and media?[/COLOR]
7. Substrate type?
8. Tank set-up (plants (live or artificial), driftwood, hide outs and other decor? - How were things prepared prior to being put into the tank?
9. Main frog staple food and any treats? How often you feed?
10. CA, vitamins, and any other additives used (how often)?
11. Lighting set-up and hours it's used?
12. When is the last time frog ate?
13. Have you found poop lately?
14. A picture would be helpful including frog and tank (any including cell phone pics are fine).
15. How old is the frog?
16. How long have you owned him/her?
17. Is the frog wild caught or captive bred?
18. Any medications in the water (treatment dosis and for how long)?
19. Any salt in water (how much)?
20. Is the tank kept in a high or low traffic area?
21. Describe tank maintenance to include water changes, cleaning, media changes, etc.).
By Lynn(flybyferns), GrifTheGreat, and aquatic questions added by Carlos(Mentat)