My cat is foaming in the mouth after I administered liquid antibiotics to her - she's recently been spayed?

Some antibiotics taste so bad they do that. It's not uncommon.

Next time get them farther back in her mouth, then tilt up hear head, and stroke her throat to encourage swallowing, She is drooling because they taste bad. And personally - there is no reason to give antibiotics after a spay, as it's a sterile surgery, so unless she had complications (pyometra?) they aren't doing a thing other than creating antibiotic resistant bacteria. Some vets are too quick to prescribe them same as human doctors.