Hello & welcome,
As you have already finished your course of Antibiotics, then it is advisable to wait & watch a couple of days to get ulcers resolved.
In mean time, you can apply topical anesthetic
gel or ointment or use any anesthetic mouth wash to get instant but short term relief from pain caused by ulcers.
Nausea that you are experiencing is of more concerned as it may be caused by irritation of gastric mucosa
associated with the use of Antibiotics.
For this, you can take antacid drugs for short period.
Avoid spicy foods as it not only will increase nausea but also hamper healing of the ulcers. Increase intake of food which is light to digest.
Drink plenty of water to avoid fatigue & dehydration
Ulcers & Nausea may occur after Antibiotics use due to gastric irritation followed by vitamin B12 deficiency
which can be treated by taking multivitamin
drugs but after examination by your physician.