Thanks for the information, I'm sorry for the delay in my reply, I hadn't seen the notification!
This kind of ulcer can appear for many reasons, such as a small irritation while brushing, a very hot food or drink, an episode of fatigue, illness or stress, and usually resolves itself in 5 to 7 days.
What you can do on your side is to use a soft wire toothbrush, and to favor a non-alcoholic mouthwash if you use it, to avoid spicy or irritating foods. You can continue gargling with salt water if you find it helps, just avoid rubbing the sores as much as possible.
A paste like Orabase can help reduce irritation when you eat, and does not require a prescription.
If the situation persists for more than 10 days without improvement, you should see a doctor or dentist as a precaution.
I hope this has been helpful to you, and if you have any further questions, please don't hesitate to ask!
Tom Samaha, pharmacist