Missed period

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I missed my period by 12 days. I supposed to get it between the 19th and 25th. It seems strange because I don’t think I got my period this month and I haven’t missed my period since I was 12/13.

I took a pregnancy test and it was negative. I had my period in may, June, and July. The last time I had sex was in may. My period has been shorter in the last 2 years. Sometime only 3 or 4 days. Anyone know what’s happening?