What Causes Delay In Period?

I got pregnant with my first child back in Sept. of 2010. I miscarried in November. I had a D&C done 3 days later. My husband and I have been trying since January to get pregnant again. My periods have been around 23 days since restarting them in December. This month, the 23 day period has come and gone with and I have had slight pregnancy symptoms similar to what I had with the first one. Today is day 28 in the cycle and still no period. I have done 3 pregnancy tests this week and one today and keep getting a negative. Should I be consulting my doctor?