What Causes Difficulty In Conceiving?

I am 25 years old and my husband and I have been trying to conceive since August, in October my period was very light, in November I only spotted for maybe 3 days, it was not even necessary for me to wear a sanitary napkin. It is now January and it has been 46 days since I had spotting. I have taken several at home pregnancy tests, one withing this week, all with a negative. I have had abdominal cramps. Up untill this my cycle was fairly regular and usualy heavy to medium flow, nothing I could mistake for spotting. There has only been one other time I have missed a period and that was over three years ago when I was taking birth control pills. I am very concerened with what could be wrong. My husband and I would love to concieve and I do not understand why this would happen right as we are trying. What could be causing this?