Spotting after hearing heartbeat on US

Jan

8 weeks today and Started spotting brown this week mainly when I wiped so I called the doctor and they sent me in for an US right away. Baby was measuring small but had a heartbeat of 145bpm. Nurse said doctor looked it over and everything looked good but still spotting brown and now more brownish red when I wipe. Don't have another ultrasound for 10 days. Just can't stop worrying.