Whats wrong with my gerbil??

[Jane Smith]

Hi im wondering if anyone would know whats wrong with my gerbil.. He is
laying on his side and not moving except for his paws and head. His legs
twitch once and awhile. He moves his arms around alot. His breathing is
labored too. Also he does not open his eyes. He also looks kind of thin and
opens his mouth. His fur is kind of matted but that might be since he hasn't
cleaned it. He seems pretty alert except the rest of his body isnt moving!
Any idea???  Thanks very much