OP I was referred to the oral people at hospital because I have a white thing on my gum, it was a white circle with red around it, not sore or anything.
Turns out it is a salivary gland pushing up and it is not harmful but I had convinced myself it was cancer. It's still there. The dentist only referred me for peace of mind.
I would personally go back to the dentist after another week or two and not the GP. No offence to GPs but they don't know half as much about mouths as dentists do and your dentist can refer you if they think it is needed. For example GP wanted to refer me for tongue lumps, but the dentist knew straight away that it was my bite and I had been biting my tongue at night and only needed a mouth guard.
A referral does not mean cancer. Your GP was probably trying to put your mind at rest and most GPs are not good at dealing with HA. Some are fantastic but others try to reassure in a way that actually causes us more harm.
If you are checking it or prodding it it will remain red and take much longer to heal. You are introducing germs and stretching the skin, it's not conductive to healing.
I bet you that if you left it alone for a whole week and looked again it will be almost healed.
I have shined a torch in my mouth countless of times and found a heap of things I thought was cancer, all of which my dentist assured me is normal for me.
So you have no reason to panic, no real medical reason that is. Ulcers can last for three weeks, they can take two weeks to start to heal. Right now you have an ulcer that millions of people all over have, a week old ulcer is nothing. A few weeks and if it doesn't heal then yes, that is more suspicious but even then it doesn't mean cancer. It could be the same as what I have, it could be a lot of things, cancer probably the least likely of them all at your age.