What Causes Delayed Periods With Pregnancy Test Negative?

hi, i am 32yrs old. Now on 3rd cycle of IUI. my LMP was 14th September. With medication my period cycle was 23-24 days.I had used Gestofit for 10 days from my IUI. my IUI was on 26th Sept 2011.There was slight spotting on 10th of Oct for two days. I didn't attend periods yet,but pregnancy tester (velocit) card result is negative. I am i pregnant else why i am not attending periods.Please advise.