The ewes lick their bums to encourage them to poo. Not suggesting you do the same(!) but wiping it with a wetwipe after she's fed may help.
Sugar in her milk will also help, now the constipation has arisen - about a heaped teaspoon in a half-litre bottle of milk I would have thought.
I also feed with the lamb facing away from me, with their body backed into me so they can't escape that way, one hand for the bottle and the other arm round their chest to hold them tight into me. The hand that holds the bottle cups under their chin to hold them to it that way.
I have a goat, who is about to kid herself any minute, but who forever more will be known as Rowan Wrigglepants - her mother died when she was about 10 days old and she did not want a bottle. She would reverse up and over my shoulder, out from my arm clamp.....somehow we got there.