Not to alarm, but as you have only had her since november.. and you say she's wormer regularly. The shape of that belly is concerning.
If I were you I'd have your vet out to do a rectal. You might have a BOGOF
I haven't read the whole read so apologies if I repeat, but in my opinion there is something else going on there. On grass 24/7 and of that type, in minimal work, I wouldn't expect her to look like that, she is under muscled yes but I would have thought she would have gained weight more rapidly... I would worm, again, keep an eye for the next couple of weeks and weight tape/condition score and then vet.