With my miscarriage in december i started with brown spotting, however i am not trying to alarm you, because alll could be fine with you & your baby!
With my current pregnancy at about 10 weeks i had like a brownish tinge to my discharge for about 1 or 2 days (only very, very slight & it started the day after sex also), but like you mentioned..... with a previous miscarriage it's hard not to be concerned! My OB's office told me that it is nothing to be concerned about also & that if it is brown that it is old blood, however i didn't want to just take the one opinion.... because i am not 100% convinced that brown means that everything is ok! I think any blood, no matter what colour should be seen to asap! SO anyhow, i called my GP also & he said from what i was describing (& i also told him that DH & i had had sex the night before it started), that it was probably just cervical mucous & that the blood was probably cervical aswell & that it was most probably caused by agravating the cervix during sex.
So i hope this has helped to put your mind at rest a little, however i do think you should go & get checked out just to be completely sure
I personally think from my experience that if it is a tinge in your discharge it is probably just cervical (a result of possibly aggravating your cervix during sex) & not related to your baby, but it is best to be 100% sure isn't it.
Wishing you all the best & let us know how you go ok!