Bunny goes green around the chin

by Bevan
(Warkworth, New Zealand)

Hello,
Help please!
Our 10 week old white bunny doe has a diet of grass and pellets, the odd carrot etc and normally perfectly happy.

We recently sold her brother, she has since lost all her bounce, thought it was just moping and waited for the goldfish-bowl memory to lapse.

Bunny is still active, but lethargic, has a green stain in the fur each side under her chin /jaws.

We originally thought it was just grass stain, but has not had it before and it's not going away. Appears to have lost condition, rather worried it is dying on us :(

Any Suggestions please?