Hi.. Thanks for the query.. I have gone through your query and can understand your concerns.. As per your complain you are suffering from Apthous Stomatitis that is inflammation inside mouth caused due to side effect of the antibiotic Omnicef.. It can also be caused due to a number of other causes like Stress, Nutritional Deficiency [Iron, Vitamin B12 and Folic acid] , hormonal changes, acid reflux, tongue trauma due to some sharp teeth or tongue biting habit, excessive intake of sour and citrus foods etc. You need to consult an Oral Physician or a general dentist in this regard to get evaluated and treated.. For apthous stomatitis you can do warm saline gargles. Gargles with 1:1 solution of benadryl and milk of magnesia can help.. Apply numbing gel over the sore for relief from pain especially during eating.. Take a multivitamin tablet for a period of 15 days.. Avoid spicy, sour and acidic foods.. In case if still the condition does not subside the physician can prescribe you to take a Steroid course as it it the ultimate treatment in case if nothing else works.. Hope this information helps.. Thanks and regards.. Dr.Honey Nandwani Arora.