Pelvic ultrasound. left ovary has 3.8cm slightly heterogeneous lesion which appears to have increased through transmission but is not anechoic. ?

The vast: majority of the time in a premenopausal woman these types of findings turn out to be hemorrhagic(or "functional") cysts that are caused be the failure of a follicle to rupture at the time of ovulation. They continue to grow, and most eventually do resolve in 1 or 2 cycles. So, follow up in 6-8 weeks usually document resolution. If it doesn't resolve, further testing may be necessary