A low placenta

So I've been worried I got a call from my doctors office saying the doctor looked over my ultrasound and saw something that worried her, it turns out she saw that I have a low placenta. I'm 20 weeks pregnant with my 2nd girl and I'm worried about it. The nurse told me that the doctor ordered for me to be rested and not do anything to heavy and I feel pain or any bleeding to go straight to the hospital.

If any you mommas have had this I would like to know your experience anything can help. Thankyou