What Causes Delayed Periods With Symptoms Like Cramps And Mood Swings?

Hi, It's the last day of the month and i haven't gotten my periods yet, it's strange because i do feel period symptoms like small stomach cramps, moodswings, etc... but i am fine, I don't feel sick, i haven't vomited or anything like that, could I be pregnant ? or can i miss a period if I've been stressed ? or I can get them early next month, I've done a test once and said it was negative and I know for a fact that my boyfriend has not ejaculated inside of me. Hope you can help me. Thank you - Ingrid