25wks here too with the same issues (and a few more), though the breating doesn't seem to be as bad as yours. Exceedingly irritating. I know when LO goes transverse as it eases the breathing, BUT stops me sleeping....as she hates me being on either side...
I vary from getting to the "shit, I need to breathe more deeply NOW" and deep breathing and puffing out to just being aware that it's more difficult, feeling like there's less room. Iron levels are fine, I'm just short (so short pelvis, leading to LO reaching my ribs sooner) and LO's growing.
Also, try this exercise:
Lie on your bed, on your back, bring legs up and feet fairly close to bum. Engage pelvic floor and bring in tummy muscles lightly. Breathing out, drop legs together (like they're glued) to one side slowly and gently allowing your sides to stretch out (roatate with your feet touching the floor), hold for a count of 4-8 (breathe IN for this bit), bring legs back up gently (breathe out), rest at the top (breathe IN), and do the other side. Repeat 4-6 times for each side. If you want to make it more difficult you can raise your feet, or even stretch up your legs
. (You're essentially rotating your spinal column so you're on your hip during the stretch but on your back when legs are back up). If you can't get your legs all the way to the matress, pop pillows/cushions under where your thighs or knees will be to stop you over-stretching your ligaments.
I know we're not meant to exercise on our backs, but the important bit is infact moving a lot of the time and I find this more helpful with my achey back, ribs, pelvic floor and stretching out my sides. Won't be stopping unless I feel feint or my bump stops me.
Also, if you think you tummy muscles have separated then it'll be worth checking out ANY exercises through your MW first.