Is Biopsy Or Follow Up Of Ultrasound Recommended?

I have had lower left back pain for about one year. Recently my doctor ordered a pelvic ultrasound with transvaginal ultrasound. The results came back with substantial fluid in the endometrium with 5.9 mm thickening. My papsmear was negative. I am 53 and menopausal (18 months). Would you recommend a Biopsy or follow up pelvic ultrasound? Do these results point toward cancer?