Are you giving your cat a pro-biotic along with the anti-biotic? Probiotics, such as acidophilus help replenish the gut with the good bacteria that the antibiotic kills off along with the infectious bacteria. The "good" bacteria from the probiotic may help with the nausea.
Ginger capsules may also help. The average size cat can take one ginger capsule up to four times daily. Place a small amount of olive oil or butter on the capsule to make it slide more easily down your cats throat to help with administration.
Probiotics and ginger capsules are widely available over the counter at health food stores and online as human products. The dose for probiotics for your cat is 1/8 of a human dose twice daily. Ginger and probiotics can be combined safely.