Bleeding at 7 weeks

Just went to the bathroom and I had some dark brown blood on the toilet paper when I wiped. I  am 7 weeks tomorrow, I had a ultrasound on Thursday, saw the gestational sac, egg yock and found a heartbeat of 119. I am just worried after having a miscarriage September 2015. When I called the ob's office they told me that it it is probably from work cause I work in merchandising and lift and move heavy items all day. But to call tomorrow and make a appointment to get another ultrasound. Any advice from anyone or anyone else going through this?