I would make sure that the lamb gets more milk from your bottle. It takes time but make sure she gets enough. Try different teats, squeeze the bottle a bit, try side of the mouth rather than the middle, different positions to you - under your arm, between your legs etc. Last year one lamb would only accept the bottle if he stood to one side of my legs(as if they were his mothers legs and the bottle was in the same position as the mothers udder). Whatever it takes, you now need to get that lamb full, 5 weeks of less than adequate nutrition is long enough.
Edited to add - is she nibbling straw bedding that may cause the bloat.