Can Abdominal Cyst Cause Mild Flow During Periods?

Hi,I am a 28yrs old woman with regular period,i have a 13month old boy,i breastfeed him for ten month but last month and this month i took notice that my period has change from heavy flow to mild flow and especially this month it's very light.i was so sure that i was pregnant due to the time i have intercourse with my husband and the symptoms i had.i went for an ultrasound abdomen complete and they found out that i have a cystic 77mm but i have to do an exam again after some month through my uterus and my bladder.sorry for my English am an Italian.