I read somewhere that if an ulcer in your mouth does not go away
after a month, it's possible that you have some sort of mouth
cancer.
Better check, it's not a good sign.
Taken from the link:
It is very important that you see your GP if your ulcer has
lasted for more than three weeks, or is abnormal in appearance -
for example, if your ulcers are made up of large red and white
patches, which often bleed. This is because in a small number of
cases, a severe, long-lasting mouth ulcer may be a sign of mouth
cancer. You are more at risk of getting mouth cancer if you
are:
If mouth cancer is spotted early on, the chances of a
complete recovery are good. This is why it is always important to
have regular check-ups with your dentist, as they will carry out a
thorough assessment of both your teeth and mouth, and will be able
to spot any possible signs of mouth cancer.
http://www.cks.nhs.uk/patient_information_leaflet/mouth_ulcer#