Concerned. Dark red/brown blood and slight cramping

Natalie • Married 11/14/15. 4 children. Mom of twins.
Should I be really concerned about occasional dark red/brown blood? Slight cramping but nothing crazy. I'm freaking out but my OBGYN says not to worry. I am 6 weeks and 4 days today. Saw the baby and the heartbeat on Monday at 6 weeks and 1 day. My husband and I have been praying and praying for this baby and this pregnancy. Any help or advice would be appreciated