What Causes Delay In Menstruation Other Than Pregnancy?

I normally have a regular period. I didn't have one for 3 months. I went to see my dr. and was given provera to take for ten days. I had a pap and everything came back normal. This was in June. After then ten days I started, but it was lighter and shorter than normal. I have yet to have a period since. I have taken pregnancy test and they have been negative. My husband and I are trying to have another baby. I am not sure what I need to do next.