Weak stiff lamb

I have a lamb that was born a week ago tomorrow and we ended up bringing it in the house to be bottle fed because the mom didn't take care of it at all and it was starting to get really lethargic. It's really stiff when it walks and it seems to have a hard time getting up off its front legs. Kneels when it tries to stand up for a long time and arches its back like it's constipated but we have noticed it poop... any ideas?

Weak stiff lamb