James, I think you're getting a feel for your rabbits.

Your 'gut' or whatever you want to call it, told you she wasn't quite ready.
How close is she to the buck's cage? I'm not too sure how you have your cages set up (I know they're under a nice breezey shade tree) but if it's possible I'd move her cage (or the buck - whichever is easier) until her cage is near to the buck. Watch her behaviour. She'll tell you when she is ready.
So if she was three weeks old the beginning of March means she was born in early February. So, she's coming up on seven months and will be about eight months old when you're ready to re-breed the others. She will definitely be sexually mature then. I think it's an excellent plan to wait and breed them together.