If you are worried, I agree, it is probably good to have a chat to your doc.
I had a similar thing at around 5 weeks: a couple of light brown smears - I thought I was having a m/c because I had mild cramping too. I don't know what it was, but everything was fine - possibly all due to the implantation process...? Maybe someone more medically inclined can be of more help! But my pg progressed without any difficulties. Hope yours continues to do so as well. xx