On bottle feeding, a lamb which has learnt to suck on a ewe will need your bottle-feeding technique to be spot on. Try to put the bottle and teat at exactly the height and angle the mother's teat would be.
Maybe try different teats and flow rates too.
The lamb may not take well to being constrained while you try to feed it, very hard I know when it isn't feeding voluntarily! I tend to position older lambs facing forwards, with their body between my calves, gripping lightly, and the head in front, then lean over bringing the teat towards them from the front. Sometimes sitting down and laying them across your lap with the back legs hanging down works better - but make sure you get that tilt (just above horizontal but not too near vertical) of the head.
They can take a while to adapt, just keep offering several times a day and don't get into a fight about it. They nearly always come round

Also she seems to be straining to go for a poo, but in her pen there is plenty of poo, so I'm not sure if she is constipated or not.
Check her openings. Some lambs are born with a blind anus, an anus opening into the vulva, or other strange arrangements. Is she definitely managing to pee?