What Causes Delayed Period And Tiredness While HPT Is Negative?

im wondering if i could be pregnant. i have a 6 month old son n have not been on birth control since, iv had a period every month since giving birth, i know when im ovulating and this month had sex the day before, my cycle lasts anywhere between 26 and 29 days, im 2 days late and taken a hpt which is negative. iv been extremely tired and suffered with nausea as well as stomache cramps the day befor i was due. could i be pregnant or could this be hormones?