Gaining instead of losing

I weighed myself last night and was shocked to find I gained ten pounds in the last month! Yikes! I should be losing pounds, not gaining it. My sister heard me shrieking and when I told her what I was shrieking about, she said that we should both go back to the treadmills and start the same rigid exercising we did a year ago. When we both lost twenty pounds each. And I should really. I cannot afford another pound added to my weight!