I expected my period the 15th, I didn't get it until the 18th, and it only lasted 2 days. However I had a day of bleeding days before on the 13th that seemed odd. I took a pregnancy test on the 23rd and it was negative. But now, I'm having severe backaches and headaches, and I just feel really off. I feel like I bled too much for it to be implantation bleeding, but normally I have heavy periods and this wasn't. What do you think? What should I do?

Wait till about the 5th and test again ( you tested too early) use first urine of the day, as soon as you wake up.

If still negative, wait a week and do again

Implantation bleeding is sorely misunderstood. Something as small as a pencil dot, or smaller, implanting into your uterus, is not going to cause any noticeable bleeding.

There are many reasons us women have an off period or skip one altogether once in a while. As of now, all you can do is wait to see what happens with your period next month. Do not test until your period is two weeks late, at that point make sure you test using first morning urine.