Late/missing period after sex but not pregnant

Hello Ladies was just wondering if anyone has experienced this. Having sex and then your following period is reallyyy delayed however after taking a test it says not pregnant. This has happened be me on two occasions now the first time it happened when my period eventually came it was super painful and the blood was really dark in colour.

Just wanted to see if anyone else has gone through this and maybe get some answers as to why this might be happening xx