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You most likely have an abscess on the left bicuspid. For whatever reason (decay, trauma, idiopathic, etc.) the nerve of this tooth has died without the usual painful signals that accompany a dying tooth. The infection that has resulted from the presence of the dead nerve tissue has built up enough pressure to burst through the side of the gum, which is the visible pimple like bump you see. The infection now will not build up pressure as it is constantly draining which keeps you from having any pain.
You do not need immediate attention but you also should not put off having the tooth looked at and treated. It will most likely need a root canal. Plan on making an appointment at your earliest convenience with your dentist. If you want a time line I would say get this treated in the next two weeks. The longer you wait, the more the infection slowly dissolves the bone away from the root of the tooth and the longer it takes to heal once treated.
I hope you find this information helpful. If you have any concerns regarding this issue that I have not addressed, please let me know and I will get back to you with more information.
George McKee DDS