Bleeding question. Help!

Em

So, I am 16 weeks pregnant. I went for my 16-week check up on Thursday, they said everything was looking GREAT! But the next day, I started bleeding, bright red blood. It was more than just spotting, you could see it when wiping and see it in the toilet, I called the OBGYN and they said to come in, they did an ultrasound and said everything was looking really good and they saw no reason for me to be bleeding. They did see blood in my cervix on the ultrasound though. After talking to the doctor, she said she had no idea why this was happening and that we were going to keep an eye on it. I’m just wondering if I should be freaking out that something could happen or feeling better knowing that the baby is okay and it’s a mystery. I’m lost. I mean, has anyone had this happen before or have some theories as to what could be happening?