Hello there, It looks to be gingival (gum) abscess
. It is a pus containing sac like structure caused due to trauma to the tissue or bacterial infection of gums. If not treated in time it may advance and progress to the underlying bone. Treatment for this is very simple, pus sac is probed and drainage
is done or a simple incision can remove the bump followed by a course of antibiotic will give relief.consult your dentist
soon.i hope this information helps you,take care.