I think that your OB/GYN is right, as you can get PMS symptoms from anytime two weeks before your actual period starts. Although a lot of women have irritability, weeping, tender breasts, fluid retention and bloated stomachs, it is possible to have stomach cramps that are like normal period pains. These stomach cramps may possibly be a little worse due to your PCOS.
There are things that you could take to help though including Vitamin B complex plus B6 supplements, Evening Primrose Oil caps and the Nutrica PMS formula, that give you everything that you run short of before your period and at that stage.
If this does not help your stomach cramps, then drinking chamomile or peppermint tea can be very helpful for this and also laying down with heat pads on your stomach can help this type of pain a lot.
Apparently, there is a new type of heat pad that has just come onto the market that is specially designed to deal with period pain. It is one of these small self-heating pads and will just slip unobtrusively into your pants so that it lays close over your stomach and will stay hot for 8 hours to provide relief.
Why not give this a try. I am going to try and get hold of one for my next period to try it out, as I always have really bad stomach cramps for at least a couple of days.