Gained all the weight in first trimester and nothing since, is this common?

I’m 35 weeks now and haven’t gained any weight since week 15. I gained 15kg in the first 15 weeks (around 33lb) and then lost 3kg (~7lb) in the second trimester. Since then my weight has stayed exactly the same.

I’m struggling to understand what the weight gain actually was that early on - was it pure fat? And does that mean I will struggle to get it off once baby is here?

I haven’t read any stories that are similar to mine so just curious what other people’s experiences are as I know the vast majority seem to gain most of the weight in the third trimester and then lose a lot of it in the weeks after baby arrives..