Late period...help?

Hi guys, sorry im just a little stumped atm. I am usually pretty accurate with my period and its never been late maybe sometimes a day early but never late. My partner started ttc last month and this is the firdt time my period has been 3 days late. Ive had mild cramping and tender breast. Have taken a few pregnancy tests but all negative. Im getting a little worried because my cycle is now 36 days long and just a little confused. Any information or advice would be appreciated. Sprry i know i sound like an idiot but im really lost....