Brown discharge after threatened miscarriage?

Currently 7w5d, early friday morning I had a little bleeding followed by spotting so I went to the ER (my OB office wasn't open being that it was the middle of the night), they ran blood tests and did a pelvic exam and finally a transvaginal ultrasound and I got to see my baby for the first time- measuring perfectly and heart beating. They officially diagnosed it as a threatened miscarriage (I was told that's what it's called anytime there is any bleeding during pregnancy). I was told to follow up with my OB since I had my 8 week appointment coming up just to see if she'd want me to come sooner, I called but she never returned my call. I was fine all day Saturday, but I've had brown discharge on and off since Sunday morning. I was told not to worry unless I had bleeding that was filling a pad/panty liner every hour for 3 consecutive and in pain, neither of which I'm experiencing, but I'm still worried. Could the discharge just have old blood caused by the vaginal ultrasound or pelvic exam? Could it be impending miscarriage? (I would call my OB but it's 3:12am- but I am going to call first thing in the morning). Just trying not to rush to the hospital thinking the worst.