What Causes Delay In Menstruation Other Than Pregnancy?

hi der,im 27 year old married women. me and my husband had unprotected sex several times during the month(in my fertile days too) i experienced some spotting and cramps. and since last week im experiencing off and on cramps and nausea.
i usually have cramps before my periods too. my usual cycle used to be 26 days but since last two month it has gone to 28 days. if im not pregnant im xpecting my periods in two days but.my cramps are still there is there and chances of pregnancy