please help!!

Hey everyone! So I’ve got a little bit of a problem. I’m supposed to start my period five days ago so I’m currently five days late which is not really normal for me I’ve never really had a regular periods (Maybe like once or twice), but I’ve been having some lower stomach pain mostly in my right ovary area. I’ve also been having headaches and back pain. I’ve also taken two pregnancy tests and they came up negative. I took a plan b 4 weeks ago so that may be the reason but I still would like to know what could be wrong if anyone has any advice pls let me know