Hello. Welcome to Just Answer and thank you for the question. I am Dr. Sohaib, a Medical Internist (who deals with diseases of all the organ systems of body) and i will do my best to find an answer for you.
Well, there are few possibilities here;
We can not rule out an STD responsible for this red patch/ulcer like lesion, however, this seems unlikely.
STD's like syphilis can give you such a lesion (but this is painless and progresses with time). HIV is also very unlikely with one such encounter and this is not the classical presentation of HIV. Herpes causes lesion or sores maily on the lips, but can also occur with in the mouth, in the form of small vesicles and multiple lesions.
Therefore, if not already you can discuss with your treating doctor for an STD screen, which can help rule out any such possibility and will put your mind at ease also.
There are numerous other causes for similar lesions, such as Canker sores (white with red borders), Erythroplakia (associated with alcohol, smoking and tobbaco chewing), Leukoplakia, Candida and Lichen Planus.
Since this lesion co-incides with cold, most likely this is canker sore (as they can occur during periods of physical or at times even with mental stress).
Honestly this does not sound something serious, but since this has been present recurrently for some duration, its imperative to have this examiend by your doctor, discussing all the details i have mentioned.
Very rarely, if a clinical diagnosis is in doubt, your doctor may take a biopsy of the affected area for histopathological studies, which will help diagnose the underlying cause for this with certainty.
Until then, you should avoid smoking, alcohol, & spicy foods.
Maintain a good oral hygiene.
Once a specific cause for this is identified, a targeted management plan can be instituted.
I hope this clarifies and answers your question.
Wish you good luck.
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I would have been worried too if i were at your place but let me reassure you that this is a treatable condition and carry a good prognosis with appropriate management.