What Causes Positive Pregnancy Result With History Of Partial Hysterectomy?

I had a partial hysterectomy in 1991. Had a CT scan and ultrasound. Couldn t find my ovaries. I have had blood work done at 2 different hospital s. They both came back with high HCG levels. Positive pregnancy results. There is no way I could be pregnant. Have not had intercourse with my husband or anyone else in 5 years. What could this be ?