What Causes Delay In Periods And Lower Back Pain Inspite Of Having Safe Sex?

I m almost 38 years old. It s been 39 days since my last period. In January, I went off of birth control pills. I took a pregnancy test 2 days ago, and it was negative. My husband and I continue to have protected sex. Yesterday I started having severe lower back pains. I m an active person, and thought this could be muscular, but it s continuing today with little relief from anti-inflamatories or pain relieving patches. Also, I am having short hot flashes. Are these things likely to be connected? How worried should I be and what should I do?