PLEASE take this with a little grain of salt, as I see it's in the Family Planning forum and my story isn't about TTC. I also don't know anything about charting. I hope it's nothing for you.
I also had the same exact thing happen two weeks ago, for the first time ever in my life. Both before b.c. pills and of course ever since I've been on them, I have always had a regular very consistent period. But two weeks ago yesterday I got my period in the middle of the month. Very very very odd and worrisome. I called my sister a day later - she is an O.B. She told me to wait to see if it happened again the next month and became a pattern but then when it continued (a total of 8 days) I called her again and she suggested I go in to see someone.
I managed to get a next-day appointment and went last Thursday. Got blood tests, pap smear, exam, and was scheduled for an ultrasound this past Monday, which I went to. I just (right before I saw your thread) called the doctor's office to see if results were in yet and they said they'd have the doctor check and call me back. I am really hoping it's nothing.
My sister had said there was a possibility of polyps (which could be benign or not). The doctor I saw also asked if I could be pregnant which would not be good timing for us right now so I said I hope not. I took a HPT this weekend to make sure. My period was due yesterday and I did not get it (my sister had said I should get my normal period at the usual time). I'm waiting to hear back from the doctor so that when I call my sister again I have some more news to give her.
Anyway, I'm sorry I don't have any further information but I wanted to share my story because I think it is important to get it checked out right away (and you are). I hope it's good news for you!
P.S. After just rereading your post, I wanted to say that I have not had tender breasts so again hopefully that's a good sign for you!