weight

So I was underweight at the beginning of my pregnancy (126lbs) and then lost more weight during my first trimester. Started gaining quickly around 25 weeks and now at 34 Weeks I've gained 47lbs total. My doctor told me she wanted me to gain weight since I was under but now it's time to slow down but I honestly don't know how. I can't really be super active because it makes me cramp and leaves me in pain so I just go walking every morning-which still hurts to be honest and I don't eat anymore than I normally would and my snacks are always fruits and veggies but I've gained 3-5lbs (it fluctuates) in 4 days. Anyone have any tips? I've been feeling kind of down about my weight lately and now hearing it from my doctor too... it's really bugging me.