It's common. It will go away on it's own. I think they call it a "functional cyst".
I found out that I had one on my left ovary at my 1st ultrasound. My doctor was not concerned about it ar all.
I had one on my left ovary. When I was 5 weeks it hurt so much I thought the pregnancy was a tubal. My baby girl is 13 days old today. It's normal. It's develops from the increase in hormone secretions.
Same thing happened to me with all 3 of my kiddos. It went away in its own every time. No worries!
It's supposed to be there. The corpus luteum (cyst that forms where the egg was released) produces the hormones that sustain the pregnancy until the placenta is mature enough to take over. It would be problematic if you didn't have the cyst in early pregnancy, so it's definitely no cause for concern.