You will be out of kilter not just with the growing lamb, but also clipping the ewe, and if you wait until July flystrike will become a serious problem... also if she lambs outside and the mucky backend just after lambing would attract flies I am sure. Also you would have to get her boostered much later to make sure lamb gets all the antibodies in the colostrum. I would separate asap and forego a lamb from her.