My period is late

I'm usually on a 28 day cycle for my monthly period but lately it has been arriving every 20-25 days; and so far, for the month of August it's late. This has happen to me before and I took a pregnancy test but the result was negative. I've also been experiencing that my body has been feeling off (not normal) but I'm not sure what it means??? Do you ladies have any ideas? And how long should I wait for my period to show? How long do I wait until I take a pregnancy test?