I also meant to add, since you were underweight you want to put even more weight on. You're expected to gain 35-40 pounds if underweight, 15-20 (not too sure on that one) if overweight, and 25-35 if a normal weight.
Your weight gain is fine. I wouldn't be concerned about it at all. Maybe if you had gained 30 pounds or something that would be a little more concerning, but your gain is fine. Even getting 1.5 pounds in a week.
I wouldn't worry about it. I have weeks where I don't gain at all and then the next week I'll gain 2 pounds. The closer you get to the 3rd trimester the more steady the weight gain will be.
I know if you start at a healthy weight the expected amount to gain is 25-30lbs. Some women to tend to gain more then that though. Also if you were underweight you should gain 30-35lbs. The last 4-6 weeks of pregnancy you can expect to gain 1-2lbs per week.