I'm wondering if it could have been a blood clot that formed where the placenta attached. A friend of mine just went through something similar. It can start to bleed it's way out causing you to bleed and spot. Sometimes it's behind the placenta where it's hard to see via ultrasound.
I'm glad that baby's doing well though. Hope the bleeding stops soon. Welcome, btw! :)
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I don't have any experience with this but wanted to send you P&PT that everything is okay.