....every four months? I'm sorry, I'm a bit speechless but I'm super glad that you've joined here.
Just for a future reference I need you to look at this thread: http://www.bettafish.com/showthread.php?t=115758
I suspect that this happened: when tanks are not cleaned weekly or somewhere around that point, many many things can build up in that tank that are harmful to your fish. It is likely she had gotten an internal parasite infection and struggled with it for a long time. Finally she's expelled the parasites but given her age, it seems like it's left her very weak after a long battle. It would be comparable to a 50 year old getting cancer, battling it for 10 years to come out exhausted and with a not good survival rate.
I'm really sorry to be blunt but every four months is not adequate, I have a feeling that it is just your girls time since she is quite old.
In the future though, if you decide to get another one, please invest in a small siphon and clean the tank out at least once a week, twice weekly is optimal so that things like what happened to your girl does not happen again.
All that said, your girl can and might still fight through this. Keep her water level lower but in a smaller tank she's going to need a water change more often. What size critter tank is it? Also feed her only 2 pellets a day, older fish don't need to eat as much. Keep an eye out for her poo though.
EDIT: alright, you can add a little more water than that, but it does look like it's her time. Keep an eye on her is all and keep that tank clean. You should change her water every other day if she survives like that. It also looks like she's got Ich if mine eyes are not deceiving me.