5000-6000cal sounds like a LOT!,.. but 2800 sounds a bit too light.
each person has their own BMR, but...
as a comparison, i'm 20 years older, 83kg and 2-3 months in as well, and 2800 is my
maintenance calories with
NO exercise..
like Dave said, eat more.
check your macros (protein/fat/carbs) and see if you are hitting them every day. if in doubt, eat more... you have plenty of years left to cut
it could also be that your form has something major holding you back?
read up, watch youtube vids, and/or go and see Trent at PTC for tips in person?
at the end of the day, we're only a couple of months in and our noob bodies are only just starting to get used to it all
eat, be consistent in training (but listen to body if it hurts), and the weight will come eventually, but your body (and all those little stabilising muscles) have to get used to things first.