As long as gynae says no issue during ultrasound, then should be no issue. I’ve had little weight gain, less than 2kg, since pregnancy and I’m in my 18th week also. I didn’t have any bad morning sickness, whereby I keep vomiting although my appetite was reduced in my first trimester and has only recovered up to 70-80% of my usual before-pregnancy appetite. I hope to keep it that way as too much weight gain is not good and hard to reduce after you give birth. Most important is whether baby is growing, not just how much weight you gain. You may gain 10-20kg but baby may still be around 2-3kg when you give birth. Then, you’re stuck with the weight after giving birth.
Yes it's normal. I gained weight at around week 28+ and bump suddenly get bigger.