hormonal effects of stopping breastfeeding?

[Lisness]

DS is nearly a year old and I have eventually (within the last couple of weeks) completely stopped breastfeeding him. This was after a long drawn out move to bottle and he is absolutely fine with it all! However, I have been feeling really rough both emotionally and physically over this period - very low and emotional (and not particularly ABOUT this) and have been plagued with more "headachy" days than not. Is this linked to some sort of hormonal realignment or is it complete coincidence?