Light brown spotting at 23 weeks

I know there's tons of threads, but everyone says it is old blood making it's way out. But my husband and I haven't even had sex in at least 2 weeks..nor has anything been inserted to bother my cervix or anything. So...old blood from what? It's the first time this has happened and I just woke up with it. Is it normal to spot light brown without anything causing it and everything is ok?