Period or premenstrual cramps are usually the culprit. Just to scar you mentally for the rest of your lives, I will rely my friend's description of how it feels when a girl is having her period:
"It feels like you are constantly peeing warm jelly and have no control over it especially when you stand up from sitting for a long time. That pain in your stomach feels like someone has rammed a rusty metal spoon inside you scrapping your insides with the jagged edges of the spoon. You constantly want to pee and wonder if that sudden "woosh" will cause a leakage problem."
On top of all that icky gross stuff, there's also the problem of acne and bloating from the hormone changes. The worst thing for a guy to do is raise the question, "Did you just have your period?" That will only make you the biggest asshole in the whole wide universe (including parallel universe) of the century.
So what should you do instead?
Well instead of fighting fire with oil, you can do these following steps to get your sweet honey back and earn some brownie points along the way:
1) Skirt around the question if you haven't confirm on the period issue, and wow your girl with her favorite comfort food or something hot and sweet like hot chocolate and cookies (hot sweet things tend to soothe the stomach cramps and calms the hormone changes). If she ask what's the occasion, just tell her you want to pamper her a bit.
2) Complement her, that's a given. Anyone feeling down can use a complement here and there.
3) If her period hasn't come yet but it's on the way, just bed her already. It'll take the edge off and you can enjoy some crazy good sex. It will be a good while you won't be having sex til the coast is clear.
4) If her period is here, offer her one of those heat pad tummy warmer and some Motrin along with comfort food. This will earn you lots and lots of brownie points.
5) Last but not least, just suck it up and take one for the team. The girl has a period, not amnesia. She'll remember what she said or she did even if she denies it, and make it up to you later when her raging hormones subside. That's when you can tell her about your new purchase of the limited edition of Iron Man suit you've bought on eBay. That or you can save that later as a jail free card in case you forget your anniversary that kinda thing.
Think warm fuzzy!