Thank you for the additional information. The picture is pretty good for how hard it is to photograph the inside of your mouth ;)
It looks like a cyst. This can be caused by a large number of things such as a foreign body (bone sliver, food chunk etc) getting stuck in the tissue, infection, anatomical abnormality etc.
A visit to a DDS should easily be able to get to the root cause of this lesion and get it fixed.
I very much doubt that this, given what you have posted, is anything scary like a tumor etc.
But, since this has been going on for over a month it is obvious that the bump is not going to resolve on its own.....so get into see your dentist.
Does this make sense to you?
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Dr. Rick MD FACS