I am in my third trimester pregnancy. I just feel my bump doesn't as big as other preggers but doctor said it is ok. But still, i'm litle afraid, is this normal?

It is fine as long as baby is gaining weight. It is actually not good to gain too much weight during pregnancy. My first baby I gained a whooping 20 over kg and my bump was very big but for my second baby, I only gained about 10kg and my bump was almost not there. He came out at 3.2kg! So if doctor says that it is okay, it definitely is. Not to worry.