Sometimes you can get oedema, or fluid swelling in response to hormonal changes as they get close to lambing. If eating, drinking and it doesn't get in the way of her vulva where the lambs will come out, she should be fine.
If it is poking out from the hole where the lambs will come through, it may well be a small prolapse which may need replacing and holding in place with a stitch, harness or spoon. Have a feel, and if it is pushing out from her insides, then it is a prolapse and needs treatment by yourself, a friendly local sheep farmer or the vet. If it is entirely on the outside, then just keep an eye on her as it may mean she needs assistance lambing.