Just need to know why i havent started my period?

Okay in MAY i started ma period tha 27th nd got off tha 31 but my cycle is usually 28 but lately its been 31 but i havent started ma period and i been so consinpated think thats how u spell it but i been gaining weight nd jus wanted to know if thats tha reason i have started but i have never went a month without a period i jus wanna