What Causes Delay In Periods?

I had my periods on 24th July 2010. Then had sex with my husband on 31st July. On 3rd August I had some spotting. Could I be pregnant? My cycle is 30 days and till now I haven't got my periods. Also since my last periods I had almost no cervical mucus. There was very less mucus for just 2-3 days last week and the rest of the days it was dry. I got negative reading for my Home Pregnancy Test yesterday. I have got a 3 year old son already, could this be my 2nd pregnancy. I don't have any pregnancy symptoms till now except the spotting.