Cyst During Pregnancy

Hi ladies,
This is my third baby. Yesterday I had my first ultrasound at 8w1d.  The baby is great, but they found a solid-like cyst about the size of a tennis ball on my right ovary.  Now I have to go back for more ultrasounds so they can determine what caused it.  I haven't ever had a cyst in my life so I'm a little freaked out by this.  The doctor said it is common for pregnant women to develop cysts, although I never had them during my first two pregnancies.
Have any of you experienced this before?
Thank you for any input.