I Am 60 Years Old And On Premarin (1/2 Tab)

I am 60 years old and on premarin (1/2 tab) for 21 days per month due to atrophic uterus and no good night sleep at times. My whole abdomen ultrasound showed that a hypoechoic mass is seen in the fundus measuring 3.4 x 2.4 cm. Is this an effect of premarin? Will it get bigger if I continue taking premarin? Is it cancerous?