Sorry to hear about your loss, difficult to say what might have caused it from that amount of info but an infective cause would be very likely if the timescale was very short.
I would also get the gender rechecked as, we had 3 rabbits who all turned out to be male, contrary to what we had been told, and they get on great until 4 months and then start fighting pretty viciously and had to be separted.

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John
AnimalLoversWeb.com

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