If you're concerned, call your OB. While it's great to get advice on here, it's usually a good idea just to give a quick call to your OB.
Usually it can happen in the first trimester especially if your monthly is normally due about this time and or if you have had intercourse! Not to wry but if you feel like it's something more like a blob of blood I would call just in case to ease your mind....
It is normal to spot after having sex and that it normal for it to turn brown and that means it is ending the spotting. The ob will tell you to hold off on sex for awhile until you are past three months.
Every time I have spotting or anything that don't look normal I usually go to the Er . no worries stay as much relax and go get check .