Reasons your period could be 13 days late?

Hi everyone:) so my period is 13 days late and I'm getting really nervous about it, I've taken two pregnancy tests and they both come back negative plus we use protection so I wasn't too concerned about that. Are there any other reasons that could cause it to be THIS late? This is very abnormal for me. Thanks so much in advance! 
ALSO is there any way to speed up your period