Pregnancy Weight Gain !

S✨ • Married - Pregnant with my First Baby
I'm 5'3 and I usually weigh about 120 lbs. Well, since I've become pregnant, I've gained 20 lbs. I range from 135 lbs to 145 lbs. 
I'm only six weeks along. 
Is that normal ? Any excerise I can do to keep this weight gain to a minimal ? Is it maybe just boob weight, cause they're heavier ?? 
Please share your stories with me.