Yes its cal def. But you are so close to harvest I wouldnt sweat it if it doesnt fix by harvest. Some of my plants do this, once they get so close, doesnt matter what I give em they will just keep cannabalizing themselves.
As Diabolical said its quite close to harvest so don't go crazy trying to fix problems. If you want give it a full dose of cal mag next watering and you'll be ok. Don't foliar feed anything.