weight gain question

so i’ve been trying to gain weight. I had my first baby and lost a ton and can’t even get back to my pre pregnancy weight.. I know I have to eat more calories than my body burns off and it’s extremely hard. i’ve been eating a ton lately but I work retail so i’m standing and walking A LOT. my body is burning off 260 calories at least while i’m working (according to my apple watch) if not more. i’m 125.. so far i’ve gained 9lbs from 116.. pre pregnancy I was 137. i’ve been drinking ensure plus as well idk what to do. my goal was 2500 calories a day is that to low?