Even tropical bamboo takes fairly cold temperatures, and full sun, too. It may just be having a "fall" moment. Leaves on mine don't look very lush and green right now either. It's also dropping a lot of leaves as well as its boots. We've had one night in the 40's so far, and many into the 50's so it's stopped growing.
Are you watering it a lot? Mine (big variety 30ft. tall) gets no water besides rain ever. Winter is cool and dry here so it goes for sometimes a month or so with no water in winter. If you've been watering it after it's stopped putting up new culms for the year, that might be the problem.
Sandi's right, a picture would be very helpful.