I'm 30 they just found a 4cm unilocular cystic lesion containing hyperechoic material. they called it a haemorrhagic corpus luteum. is this cancerous?

No: Hemorrhagic corpus luteum is a benign cyst which forms in the ovary following release of an egg.

See answer: Hemorrhagic cysts often develop in premenopausal women due to hemorrhage within a corpus luteum. They vary in ultrasound appearance depending if they are acute or resolving. These hemorrhagic cysts are common, resorb and are not associated with malignancy.