Well, not exactly, greens are no problem if they are available all the time from the start, it's the most natural starter food possible (wild rabbits are not that different to domestics). When they start to nibble they will grow the right bacteria to digest that. Problem is when you give a kit who is used to other solid food suddenly greens - that you have to introduce very slowly. But I don't think that's the problem here.
Make sure the hay isn't moldy, no bad smell etc.. This time of the year it may be pretty old and if there was some bad storage condition it might be spoiled. Possible the bigger rabbits can tolerate more of this or eat less or no hay when they get greens too.
I'm no expert on rabbit deseases, but I've never heard of one with symptoms like that.
Hm, like Zass said, mastitis could be a problem, and the timing just coincidence, check the doe.