The main cause of ulcers in the mouth is aphthous stomatitis, i.e. canker sores. But the second most common cause probably is an initial infeciton with HSV1, i.e. oral herpes. Cold sores, i.e. recurrent oral herpes, are localized, usually just one or two sores on the lips. But the first infection typically causes highly painful ulcers inside the mouth, usually with severe sore throat as well. If you have anything like this, see a doctor ASAP for accurate diagnosis and treatment -- which to be effective must be started ASAP. If it's herpes, most likely you acquired it by kissing an infected person. In any case, acute HIV is a rare cause of mouth ulcers, and would almost never bet he only symptom. And fatigue alone means nothing, without fever and other symptoms. There is no reason to suspect you have HIV. Still, as I said above, feel free to get tested if you will gain additional reassurance from the negative result. (This does not mean I believe there really was any risk.There was not. Strictly because negative blood tests often are more reassuring than expert opinion, no matter how scientific.)