Have you tried increasing your Iron intake (especially during your period)? The flatness could be due to the loss of blood (the loss of iron in the blood in particular). I'm wary of vitamin tablets because I'm not sure how well our body actually absorbs the vitamins out of the tablets when it isn't present in a food form... But lots of green leafy vegetables (if you can manage it) like brocolli and spinach (awesome in currys and stirfrys)