Antibotics and bacterial infections can cause a reaction like a rash, so it could have just infected your mouth. Also, a white substance could just be another reaction to the anitbotics, like a yeast infection or thrash.
There are numerous lesions that can emanate within the oral cavity that need a physical examination for complete evaluation. Viral illness, whether it be from upper respiratory infections or hpv, can lead oral lesions. Obviously, the duration of lesions is related to what has caused it. In addition medications, including antibiotics, can lead to changes in the oral cavity as well. You need to be seen by a physcian to help you in diagnosis. Sometimes cultures or biopsies can be helpful. Best wishes.