A pound and half overnight is actually a very normal thing for our bodies. Our bodies are made up of 60% water, so our weight can change from hour to hour (nevermind overnight). Many different things can make you gain (hormones, too much sodium, exercise, stress, humidity, not eating enough, eating too much, etc). It doesn't mean you were not eating healthy. It is nothing to get too upset over. If you weigh ever day, you need to expect lots of ups and downs. I can go up 4 pounds one day, and be down 5 the next. It's the overall downward trend (weekly, monthly, etc) that counts. Drink some water and if you don't see a change in a few days, then analyze what you are doing. It could be many things. Good luck!