Brown discharge at 9-10 weeks pregnant TMI

I am 10 weeks pregnant, I've been to the ER because I started having this brown discharge out of no where I thought I was having a miscarriage but I wasn't, at the hospital they did blood work and urine tests and an ultrasound . Everything was good including the tests.  And my baby was healthy with a beating heart which gave me some relief.  I then made an appointment with my prenatal doctor after my visit to the er, my doctor said its common to have some discharge during the first trimester.  I'm still very worried even though I've had reassurance that everything is okay with the baby.  And I was told that it could be a threatened miscarriage.  Has anyone had brown discharge during their pregnancy ? I was told it could be old blood from my uterus since it is expanding.  Need to ease my mind I can't help to worry 😞