Nifty Expiry last minute price movement

I had sold call for nifty 27 May 14350 CE, now around 3:28 -3:29 nifty crossed the 14350 mark and by the time I realized it was 14360+, so I went to close my position expecting it to trade around Rs 10 but it was still trading at Rs0.3-0.4 (it was odd) so I closed my position and by 3:30 nifty closed around 15370 ( Rs 20 ITM).
But around 3:31 the nifty price dropped to 12337.85, so what just happened?


The price which you see at 3:30 is the LTP (Last Traded Price). The exchange determines the final closing price for the day using the Weighted Average Price of the last 30 minutes of trading. This can differ from the LTP you see at 3:30. You check this post to know how WAP is calculated.