How can I be pregnant???????????

so I got what I thought was my period last month but it lasted about 3 days, when it usually last 7 days & it was late. I have been waiting for my period this month, I should've started Monday and it is now Wednesday. I usually have a 28 day cycle...could my cycle change? Please say that is the case. I have 3 children and really do not want any more. I do NOT feel pregnant. I am usally very sick. I am hoping I am just stressed. The only symptom I have is nasuea, but I just associated that with me about to start my period. Also my tubes are tied. That is another reason why I do not think I am not pregnant. I am going to wait until the weekend to take a PT. But I was wondering what other females thought.