What Do Irregular Periods Indicate?

hi, last month, my period starts on jan 26 2010. but, this month suddenly there's a very small amount of blood coming out on 21 feb and stop on 22nd feb. at first i thought that my period for february came early than it used to be (normally every 25 or 26 every month). i never have this kind of situation before. i tried to check with pregnancy test kit but it is negative. i was married for more than a year but failed to get any child until today.