Noticed my bunny boy wheezing a tiny bit the other day so a sudden vet visit was scheduled. He is seven and had a URI last year around this time with similar symptoms to now. Turns out he was pretty dehydrated (despite seeming to drink plenty from his bottle all the time) so the poor baby had to stay for a hour and get fluids and his blood drawn to test his kidneys. Everything came back perfect, but he did indeed have a slight URI, still in just the super early stages thank goodness. Boy is now home and going to getting a bowl along with a bottle of water to make sure he has constant, easy access to water.
Remember, always check your animals and keep a eye out for any changes in behavior or intake/output of food and water! Minor things can become major really quickly!