Dark spot on placenta?

Erin • Mama to 3 beautiful girls
My doctor called me to go over the results of my 24 week ultrasound, however we keep missing each other. The nurse was able to tell me that my doctor found a dark spot on my placenta and that's what she wanted to talk to me about. She also said it wouldn't affect the baby. But I can't help but worry. I can't try to call her back until tomorrow. 
Has this happened to anyone else? What were the "dark spots" on the placenta and how did it affect you or your pregnancy?