means there is an accumulation of filtered urine at the level of the ureters just close to your kidneys. The probable cause shall certainly be a compressive effect of these large fibroids in the ureters, obstructing the flow of urine from the kidneys to bladder. Left untreated, accumulation of urine shall continue to cause enlargement of the urine collecting tubes upstream, ureters and the kidney pelvis, favour growth of bacteria and consequent infection of the kidney and lead to in a time less than could be imagine, kidney failure.
The management of hydronephrosis in your case for sure be removal of these large fibroids. Comsult and concert with your gynecologist on the management of your case. Its advisable that you get the fibroids removed (myomectomy) and this shall help for sure.
Thanks and hope this helps,