Ryno, I keep pond koi, and have done so for the past 8 years. It's not likely that this koi just suddenly got fat. She may be egg bound or she may have a tumor. In either case, there isn't much you can do. Aside from being egg-bound or having a tumor, the only other thing I can think of that will lead a fish to suddenly become noticeably bigger is Swim Bladder Disorder, but that's usually fairly obvious because the fish normally will either begin swimming sideways or, in the case of one of mine, each time it stops swimming, it float, belly up, to the surface.
You didn't mention how big she is (length-wise) but if she's mature enough to be spawning, she's almost certainly too big to be in a 55 gallon tank, even with mega filtration. Being in a tank may not have anything to do with her disorder, but as a rule, koi, shubunkins, and most any large goldfish simply do not do well in any sort of tank--at least in my experience. Good luck!
Steve