If it is Phyllostachys Nigra (Punctata) (The Black bamboo) it is NOT a runner and invasive. In fact it is a clump forming bamboo and very well mannered
My own specimins have held well in large clumps are are giving no trouble
Bamboos are said to sleep for a year, creep for a year and then leap
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I have a Spectabilis that has shot up two new shoots to over 6 feet in 3 days
Yours is probably doing just that
Be careful about fertilising bamboos, as they can become too large and may well spread more than you would want if you do