Hi.. Thanks for the query.. As per your complain canker sores can be caused due to causes like deficiency of iron and vitamin B12, stress, physical trauma to the tongue, allergies, acid reflux, side effect of medication etc.. Other common cause can be herpes virus infection causing cold sores.. If it is viral infection then it is surely contagious and if it is canker sores then there are less chances of communication of infection but still it is always advisable not to share your personal belongings with others.. For now you can start gargling with a numbing mouthwash containing Lignocaine to relieve the symptoms. Do warm saline gargles. Apply an oral steroid ointment along with Chlorohexidine ointment.. You can also take a multivitamin tablet daily.. You can take anti inflammatory painkiller like Ibuprofen.. Suck ice and do cool water rinses.. Avoid spicy food.. If there is no improvement in 3-4 days consult an Oral Physician and get evaluated.. Hope this helps.. Regards.