Apparently around that time you are most energetic and more likely to lose weight etc etc. Bounce around a little, go for walks try new things. Do not sit around and feel down, I know the feeling and I prefer to spiral into glum thoughts and get into a mood (not PMS yucky phrase)...It is easier to feeel low then do something about it.
Do not overdose on the St John's though. Happy pills do exist but try alternatives - being with others who are not feeling so low, doing things that you enjoy a change of scenery perhaps. Think bright - the cup is half full not half empty and the best is yet to come. Think of times when you were happier and try and get back to that way of feeling. Try and solve any problems if that is what is making you feel low.
With things like chocolate you get a high and then a low afterwards as well. One step forward and two steps back. Clothes/decorations etc would be good as you can see where it has gone and say well this item makes me feel/look good and you feel positive about you and doing something for you. Or maybe something charitable, for another individual.