Allergy. The initial answer has nothing to do with an allergy. An allergy presents as itching, rash. It can progress to shortness of breath and respiratory failure.
Drowsiness etc. CNS depression symptoms like somnolence, drosiness, dizziness etc.
See below. An allergy describes a systemic response of the body's immune system to an antigen. The response can cause hives, itching, swelling, elevation of blood pressure and bronchoconstriction. An allergy to metaxalone would basically be any combination of the above reactions in response to the intake of metaxalone into the body.