Originally Posted by echinosum

Are you sure that what you have is Phyllostachys nigra (proper black bamboo), or might it be Fargesia nitida (often sold by the unscrupulous/ignorant as black bamboo)? F nitida has been flowering recently, and usually dies as a consequence. Have a look to see if it has any grass-type flower/seed structures on it. How can you tell which you have: P. nigra sends up culms which are initially green but slowly turn shiny black over the course of a year or so, whereas F. nitida sends up dark grey culms which are not, or only briefly, green. P nigra has only two branches at each node on the culm(though one may have a sub-branch very close, so it can look like 3), whereas F. nitida typically has many branches at each node on the clum.

If after that you are sure that you have P. nigra, perhaps you can tell us
(1) where you live, what minimum temperatures it has experienced, what frost/rainfall has it had the past few months
(2) how big is the bamboo and how big is the pot
(3) how many years have you had the bamboo in the pot
(4) when did you last repot it or divide it (if ever)
(5) what watering/feeding have you been doing