What Causes Left Abdominal Pain And Spotting?

I am 28 yrs old ....my period is due to on 36th day & i got blood dripping & spotting on 28 th day . i am little bit confused .whether its a normal period or not ? & also i have got some prickling pain in left side of the abdomen. tested for pregancy ...but its negative ...can you pls explain me the pain