Without examination, it is impossible to say for certain, but it sounds like a developing cold. An infection equivalent to the common cold in man, with similar symptoms, is known in the rabbit world as ‘Snuffles’.

Apart from being very infectious, there is a real risk of pneumonia developing. Isolate the rabbit from others and consult your vet. If your vet finds nothing, the other option is that your rabbit’s cage substrate or bedding is inducing an allergic reaction.