That sounds like both of my "first" periods after my kids were born (and I'm so jealous on the 13.5 months of no period! Nursing hasn't helped me keep it at bay that long *pout* LOL)
Right before my period, I always get diarrhea-y. Crampy, too. My milk supply goes down, so the baby tugs, fusses and chews while nursing, which is miserable, since my breasts and nipples are tender. And, yup, it was all worse before the first one, for about a week! Then, when my period starts, my milk supply picks back up to normal within a few hours, making the baby full and sleepy, the cramps let up after a day or so, the headache goes away (I usually take to bed with a migraine the day before I start), and my mood takes a definite upswing.
Give it 24 hours and see if you don't "really" start.
Oh yeah, and there's a stomach bug with cramping and all kinds of ick going around that's supposed to last about 24 hours. If you have THAT, instead, it should go away fast.
Good luck and hang in there!