Most likely the bead is not seated. The tube is thicker at the stem, making it more difficult to get the bead all the way down. Most tires have a molded in line parallel to the bead that you can use to check if the bead is all the way down. The tube may also be pinched between the tire and rim. That can cause a lump in the tire. Let the air out push the tire over to each side. You should be able to see the rim strip and see that the tube is not in between the tire and rim.

If you did pinch the tube, it will eventually wear through and pop.