My goldfish lays at the bottom of tank

I have had my goldfish for 4 yrs. She started in a 1 gal tank but now is in a 3 gal. She was fine for awhile after the move so i do not believe the change affected her. For the last three months she does not come to the top when i feed her like she use to. She spends most of her time lying at the bottom of the tank. On occasion she swims to the top, goes from one side to the other like a dolphin(nose straight up and top part of her body out of the water) then she falls back to the bottom(literally lets herself fall)at the bottom i notice she lays a little bit to one side.

I have been reading the internet to try to help her. I just bought an airfilter thinking she needs more oxygen, starting tomorrow i think i will try feeding her peas to see if its the stomach thing. Alot of sites that describe the same symptoms say the fish is dying, but she has been like this for months so I dont know if she is or suffering from something. When my friends ask whats wrong with her I usually say shes old and had a stroke. Shes a pretty hearty size gold fish. I have a picture of my tank as my avatar to view her.
Can someone with fish experience help me know what may be going on with her.