Although 15-20 is very respectable and up there in age, silver dollars can live much longer. I would not consider her an old silver dollar, just a mature adult. I think it is just an age thing. I have a trio of Metynnis maculatus that are about 26 years old and continue to go strong. Yours seem to be a female M. Altidorsalis or M. Argenteus, and the other 3 dollars in the picture appear to be a male M. Altidorsalis, and 2 females of possibly the same species (but the picture do not show the whole fish).
Regarding color change - not that I can say I know what is happening, was she more golden and has become lighter, more silver?
Did you imply she has become fatter? As in swollen? If so, two possible things come to mind- a digestive obstruction or internal buildup, or a tumor. The former sometimes can be helped wit treating with epsom salt. The latter, has no solution, but usually looks non-symmetrical (more towards one side, or as a localized lump). You need to observe and decide and perhaps posting additional pictures can help others help you decide.
In my personal experience, a varied diet is critical to maintain them healthy, with lots of plant (various plants, seeds, algae) and animal (insect, worms) foods. Just flakes (even good quality) does not quite do it long term.
Yet another possibility for color change (assuming other things are ok), is social interactions and ranking. So a male and two additional females were introduced and that may have shifted rankings and attentions. These fish are extremely social, and their hierarchies change, even seasonally.
I have two silver dollar tanks (125g), one with 7 adult, 3 species, the other with 5 adults (2 species). Rankings change from time to time, and individual fish bossiness or submissiveness change, often involving some color changes.
Hope this helps. Good luck!
