This is likely to be due some space occupying lesion (tumour) in the pelvic region. It could mostly be fibroid
of considerable size. But fibroid should cause you irregulat and heavy bleeding.
The other tumour could be some ovarian tumour. Ultrasonography
will show if there is any lesion in the pelvic region or lower abdomen.
The other organs like rectum
or urinary bladder
also cannot be ruled out.
The other non tumourous pathology could be Prolapse of the uterus
and or other reproductory organs. However, they get relieved in lying down position.
There is no point is wasting time. Better see your gynaecologist ASAP and get properly diagnosed and treated.