Hello, I'm a small animal veterinarian. I can try to help answer your question about your rabbit.
How long has he/she been experiencing these symptoms? Where they sudden?
Is your rabbit housed outdoors or indoors?
Is there potential for injury such as a snake bite or even a wound that has become infected causing the swelling?
The lack of balance could be due to the swelling - again this could be infection related, or even possibly an allergic reaction to a bug bite.
Rabbits are very sensitive creatures in general. When they are sick, they tend to go downhill very quickly. I'd have to suggest a trip to your local emergency clinic if this is at all possible.
Please feel free to respond and we can discuss his/her condition further.