It could be the start of twin lamb disease, or it could be something else. If you have a vitamin shot, I'd give her one. Or, better still, some Twin Lamb drench. Definitely keep a close eye - check her ears aren't cold, her mucous membranes are a healthy colour, smell her breath for signs of ketones, watch that she's pooing normally, etc. If she goes down (other than to lamb), get calcium into her.
she's just getting ready to lamb.