What Could Be Causing A Missed Period Despite Negative Pregnancy Tests And Persistent Period Pain?

I’m suppose to see my period 26th of April and unfortunately I didn’t see it and this May 7th and I haven’t see my April period..., after conducting a lot of pregnancy and I find out that I’m not pregnant but yet my period is not forthcoming though I’ve been having this period pain since 26th of last money which is the suppose date my period was suppose to start. What could be the cause pla?