Weight gain during pregnancy

Pre pregnancy weight was 124 and now I’m 28weeks and I’m already 163lbs and I honestly don’t know how I am walking and staying active and I don’t even look that huge I do sleep a lot and I don’t eat a lot throughout the day just snacks and occasionally dinner that my fiancé makes me I so worried my baby is going to be big and my doctor already told me I can’t be gaining so much I gained 12lb in one month I don’t know why can it be water retainment or just from my butt and boobs getting bigger because I’m really not showing in my belly the amount I’ve gained I’m so confused I’m 5 feet and I have a small figure I have my appointment tomorrow and I really don’t want my doctor to make me feel even worse if I gained almost 12lbs again this month