In the mouth there is a multitude of bacteria, viruses and fungi that exist in a natural equilibrium. Due to diseases and antibiotics this equilibrium could be disturbed and will result in the fact that particular microorganisms proliferate and, e.g., can trigger fungal diseases.
One of the most frequent fungal disease is soor – a whitish film on the oral mucosa that sometimes will ache. Soor will be treated well by an anti-fungal drug. In any case you should have your dentist examine a change of your oral mucosa.