The appropriate weight gain during a pregnancy depends on several factors, including mother's pre-pregnant weight and age. A woman who is of average weight is encouraged to gain somewhere between 25-35 pounds during pregnancy. Underweight women need to gain a bit more weight, and high weight women a bit less.
It is VERY common to see a big weight change around the 6-7th month. I know that with both of my pregnancies, I had done so well and then all of a sudden I gained 8 and 10 pounds at each of those visits. It happens to many women at some point. It will level off during the last trimester and you might even lose during the last few weeks.
I know that with my first, I had hardly gained any weight until the 6th or 7th month and then right after I gave birth lost alot of that since it was water and bloating. So, do not be too concerned over this.