It seems as though your doctor is convinced you have a PMS related mood disorder (does exist.) However, I am weirded out over the idea that he is giving you an antidepressant to help something that seems more likely to be a hormonal issue. AD's act on neurotransmitters, not hormonal.

300 isn't the highest dose, but it is high and it may have been more appropriate to start on 150 before barraging your brain with such a strong effect.

I think you should seek another professional opinion.