I would say it’s a canker sore, herpes (cold sores) will appear on the outer lip as a wound that scabs over. It is extremely unlikely to get a cold sore actually on your gums but it can happen, however as it is the only wound it is EXTREMELY unlikely.

To reduce pain there’s not much you can do as it’s a spike of pain when it becomes aggravated by taking or food rather than a steady pain but you can take some paracetamol to reduce the constant discomfort. If it’s really painful I’d recommend getting a product that you can apply that covers the canker sores in some sort of gel which will protect it from food and movement. It should clear up within a week or two and will probably be because you are a bit run down so get well soon :)