She could be sore or even have mastitis. With mastitis the udder would be inflammed, hot, hard, tender to touch, and the milk may look abnormal. She may also be off colour herself but not always. This will need antibiotics and anti-inflammatories. If the lamb hasn't been sucking that much and she's producing lots of milk she might just be too full of milk, in which case you will have to milk some out and then the lamb will have a chance to latch on.