It could be a mixture of reasons you've gained 8lbs so far.. water weight, bloating, uterus, etc etc. I wouldn't be too concerned about it especially if you've gone to the doctors and they haven't mentioned it. If it was too excessive, they would say something but its always good to watch what you eat and make sure you're eating in small portions so you don't get indegestion or heartburn. After a meal, or before, go for a little walk, it'll keep your insides running while they are changing and getting smooshed lol :)
Check out some pregnancy exercise videos on YouTube