I usually know when Im going to have my period but then 1 cycle have changed. I usually would get them close to the end of the month, then it changed. Last month, i came on the 18. I usually come on around like the 25th or so. I am waiting to get mine now. I haven't came on yet and i've been having pains since before then. what does this mean? Also I took a pregnancy test and it came up negative.