I hate it when your pants feel too small. I find around the bloating stage of my period I just feel like crap. As a tip of how to guess when shes getting her period my boyfriend tells me that I tend to complain alot, I second guess most things that I do, I get more irritated such as every little thing bugs me (hes late picking me up, plans suddenly get change etc.) Also I find personally I am more inclined to eat unhealth foods such as fast foods (which I usual hate), chips, anything high in fat lol :P.
My headaches usually get worse and if i eat too much chocolate or caffine they get to the point of hardcore migranes.
Just watch for her to get irritated and being more of a perfectionist. I notice with my sister mother and female friends that perfectionism normally happens. Their hair and makeup needs to be perfect or they will have a mental break down haha.
...The joys of being a woman!!