Could be a start of the period coming on...or could be normal spotting around the time you would expect a period while becoming pregnant. If I were you, as it seems you are TTC I would wait until 1 week late before testing.
Good luck.
I had brown spotting from about 13 DPO until about 15DPO. On 13DPO I really thought AF was on her way by morning; however, when I woke up I noticed my bbs were much more sore than they were the day befor, so I decided to test at 14DPO and got a BFP!! I know how hard it is to test and how disappointing a BFN is but this just might be your month!! Good luck and SSBD to you!!