Need advice

So we went to the doctor today and got told I have placenta previa. Not a big deal, but then she told me at the top of my uterus there is something there. She said it could be another placenta, no big deal. Or it could be something wrong with the uterus itself. My next appointment is in 4 weeks they are hoping they can see it better then. I’m 20 weeks. So I have had 4 ultra sounds since we found out and I feel like if it was a second placenta wouldn’t they have seen that my last ultrasound, which was 3 1/2 weeks ago. Has anyone experienced this? Or know what it could be.