Have a small red bump on back of heel & has like white center. Very painful I've had it b4 it comes and goes. What could it be? It's as small as a pea

Foreign body or cyst. You have to rule out a foreign body which may have become embedded in the back of your heel, but there is more of a likelihood that it could be an inclusion cyst which can be painful. You should consider having it examined and biopsied which will basically take care of 2 things - it will remove the immediate problem and help determine what it is. See a podiatrist!
Many things. If it is on the back of the heel it could be a small blister that comes from shoe rubbing against the back of your heel. If its constantly there it could be a porokeratosis which is a clogged sweat gland or a wart. See a podiatrist and a biopsy of the lesion can be performed to determine exactly what it is.