Although it seems to be normal aphthous ulcer that occurs in the mouth I would advise you to get a round of test for STD's.
Aphthous ulcers are ulcers that form on the mucous membranes. Aphthous ulcers are typically recurrent round or oval sores or ulcers inside the mouth on areas where the skin is not tightly bound to the underlying bone. Oral sex is the act of sexual stimulation of a partner`s genitals using all or some of the parts of the oral system .They are not detrimental to health and considered a safe sexual practice although, it is notorious to cause herpes, especially if your partner was in the active phase of infection.
I would recommend the following.
1) Get a serology test for herpes infection .If not diagnosed they can spread rapidly. If positive, you will be given a dose of Tab acyclovir.
2) Do not engage in oral sex till the ulcer heals as the ulcers may get infected and cause complications.
3) Start a dose of Vitamin B12 complex and Vitamin C tablets to improve healing.
4)) always use protection while having sexual intercourse and follow safe sexual intercourse