Delayed ovulation? HELP

Lisa • Married 5 years to a wonderful man. Son born 26 Jan 15. Expecting another boy in Nov 16 X
Hi all
​I have been tracking for 6 cycles and always O on day 15.
​However, I'm on day 13 and my CD monitor is on 'low' (normally high by now and peak tomorrow) and I also tried another opk brand which is also very negative.
CM is not fertile. 
​Has anyone else experienced delayed O before and if so, how long was it delayed by?
​The only thing I have done differently this month is take 50mg daily of B6 with my pre natal vit, I stopped B6 yesterday in fear this might be the cause.
​Please help..
​Thanks