I missed my period....?

Last month I started on the 2nd and my period lat one full day with very light brown spotting the next day. I have always been regular and my periods last about 4 days. I have not started my period this month and I am 3 days late. I am not stressed and never missed before. Been trying to conceive for 7 months but i tok a pregnancy test and it was neg. what else could cause a missed period?