Rust Brown to Dark Brown Discharge

Been constipated and finally passed it this morning no bleeding at that moment but toward the evening i notice brown pasty discharge on my tissue a few hours later it turn dark brown and came out like dirt grains with my second bowel movement once i was finished it stopped and lightened up still freaked out and being i had a miscarriage before this pregnancy i went to the ER and had a u/s which showed moving baby and strong heart beat and the doc said its a normal pregnancy but didnt say what causes the discharge! Anyone else experiencing this??!