Why was I overweight?

More than likely OP, you were eating above maintenance. It doesn't mean you were pigging out or eating "bad" foods, but you ate more than you burned. I got to where I was partially because of PCOS, but also because I overate. I didn't think I was eating badly (my parents put me on diets for much of my childhood, and I spent most of high school eating vegetarian), and people were surprised that I liked vegetables more than sweets/most fried foods. But when I started meticulously tracking my food intake and measuring what I ate (starting with weight watchers and now on mfp), I found that I ate more than I thought I did, and my portion sizes and perception of what was calorie-dense and what wasn't was pretty off.

So in short, I don't think it means you need to severely restrict, but it does mean you should track consistently and honestly, and live a lifestyle (this is for life, not until you get to goal) that is mindful of what you are eating to lose or maintain, and is sustainable.