Anytime Really.
There is no difference between a bump and a lump.
A bump is a lump like, for example, a bump on your skin or a bump in a road.
If your child hit their head that hard, you should take them to the doctor right away.
LUMP
They have a bump for matting.
Bump Chump Pump Lump
bump, lump, clump, rump, dump, hump
Hump, dump, stump, bump, lump, rump.
Like a hairy lump
No there should be no lump. Due to a failure to either follow the aftercare routine or the use of improper aftercare products you may have developed a hypertrophic reaction which is causing the lump. Only a visit to your professional body piercer will best tell you what is going on with your piercing.
Depends on the nature of the lump/bump. Might be acne (yes, it does happen there), or it could be something much, much, worse, and destined to cause you a whole lot of suffering.