Some women do spot or have very light bleeding during ovulation - i've had it on a few occasions. Implantation bleeding will only last for a day max and would be that light that you might not even notice it. Also, implantation bleeding is not going to occur mid cycle - ovulation does. Which is most likely the cause. Other things can cause this mid cycle like infection or STD but again it's more likely to be a little heavier than that.
A Pink Discharge that is very little and or last for a day or two is implantation bleeding. Implantation bleeding is when the fertilized egg attaches to the wall of your uterus. I suspect that it is implantation bleeding because its before your period. Ovulating does not produce a pink discharge. Ovulating is more of a clear, watery stringy discharge.
Most likely it's because you are ovulating, which happens mid cycle.