Does it happen to you every month or just this month? It could be a one month fluke hormonal thing that won't happen again. It could also be a sign of low progesterone - in the second half of the cycle, after ovulation, progesterone levels go up to prepare the lining for implantation of the egg. If progesterone levels are low, it can cause spotting in the second half of the cycle.
A more positive thing would be implantation bleeding, although starting at 3DPO would likely be too early for that.
If it happens regularly, you can go to the doctor for progesterone blood tests (have to be done on certain days of your cycle) to have your levels tested.
Good luck!
