On 31st I thought I might need to exit and enter again since it's the 180 day since the first time I entered .
No, unfortunately, it does not work this way. The 180 days is a moving period, it does not get reset. Lets assume you have entered Schengen zone on 15th of December and will leave on 6. March 2017. Now you look back 180 days.
06. March 2017
Substract 180 days
Result: 07. September 2016
This is your 180 days period. Now you ask: How many days have I spent in the Schengen zone between 07. September 2016 and 06. March 2017?
Days of stay in the Schengen zone within the 180-days period from 07. September 2016 to 06. March 2017: 87 days
So, as of 06. March 2017 you have only 3 more days left for your stay in the Schengen zone, then you have to leave. You will be able to return after you satisfy the 90/180 rule again.