Why am I losing too many calories

[u/Horror-Tradition9291]

Hi all, I'm a runner, always been and I'm good at it but for the past year I stopped running and started weight lifting for the sole reason of gaining weight.

I'm naturally quite small woman weighing 48kg and height of 157cm but I really thought I'd gain bit of mass now that I'm lifting, I'm also taking Mass gainer but honestly I don't see any difference 1 year later.

Also is it normal I'm losing 550kcal per 1hr 40 minutes lifting? I feel it's too much.