Cramps but no blood

On the scheduled first day of my cycle, I had very light cramps, more like back pain than anything. They only lasted the day, and I thought it was my body warming up for my cycle, so to speak. However, there have been no cramps since then and no blood. Does it sound like anything serious or just an off month? (I had PROTECTED sex a few days before my period, but I'm positive nothing broke, slipped, or spilled!)