Racing is confusing on Zwift as they now use two different categorisation systems to decide what your racing category is.
The original system (still in place) is generally called ZwiftPower categorisation (ZP). (Average of your best three races, Edit - group rides and events, in past 90 days)
The New system is Category Enforcement (CE). ( your best activity, racing or not, in past 60 days )
You are a CE cat C probably as a result of your last race.
But the good news is you are still a ZP Cat D as your last race does not take your three race average over the limit.
If you wish to continue racing in D cat all you need to do is find and enter ZP enforced races.
Picture below shows you the ‘CE categorised’ symbol ( warning sign !). You need to avoid these races and choose those without the symbol.
So some good news.
But the bad news is that the ZP system allows anyone to enter your D category race as it doesn’t force people to ride in their category.
I’m sure you are aware you can also race as a D category in Women only races.
I have also provided two links, both long but probably just top post in each link the most useful.
Race on - in Cat D 
