I'm on a quest to trim and slim but I stalled ... WHY ???

I put myself on a work out regiment that entails 45 minutes of cardio and toning. After a month of working out, I averaged two pounds a week in weight loss. I all the sudden stalled. I started drinking two liters of water a day...then lost two more pounds and stalled. My friend suggested that I cross train. Will this help? Why have I stalled?