First you should find out what the lump is, it may be an abcess. Often caused by plaque, tartar, food debris, faulty filling or crown, or brush bristle.
It depends. If the lump is hard it may be an erupting tooth. If it is soft and painful it may be an abscess. If it looks like a blister it may be a mucosele.
A bump on your gums next to a tooth can be an infection draining out. This is called a parulis. Your dentist will check the teeth nearby this lump to see if any of them are infected and will take xrays to check. A root canal will fix this.
If it is painful then it could be a pocket of infection, or an abcess tooth. It could also be a pertruding tooth. Either way you should see your denist about it. A lump on your lower gums is not normal and needs to be checked out by a professional.