I took Lexapro for 8 months, and during that time I gained 30lbs. I was being really carefull about what I ate and I was getting tons of exercise. No matter what I did I couldn't stop gaining weight. It was great for my anxiety, but the weight gain was just too much for me to take, so I complained to my doctor about it and he switched me to Buspar. That was about a year ago, and once I got off Lexapro I started losing weight immediately. For me, Buspar works just as well as Lexapro did, but with no side effects. I think I'm one of the lucky ones, though, cause I've heard that Buspar doesn't work for a lot of people.
Anyway, yes, it probably is the Lexapro causing your weight issues. When I was on it I found a lot of websites with people complaining about weight gain because of Lexapro and other SSRIs.