Could be finished blooming - my Cym blooms usually last around 3 months, tho plants are outside in cool temps (last longer in cooler temperature) - if all the blooms were open when you got the plant there is no way to know how long it had already been blooming.
If it's in the path of warm air heat blowing, that could cause blooms to prematurely wither.
It would be difficult to get a Cym too much sun in a window, they like a good deal of light.
It's also difficult to rot growth on Cyms (in my experience anyway), so long as plants aren't standing in water for long periods.
From what I can see, your plant looks good