Why is my boob unusually hard?

Normal engorgement? In the first week, it is not uncommon for mothers to develop engorgement of the breasts as your milk is just coming in. Another possibility is that the milk ducts may be blocked. Try a warm compress and massaging if you feel any hard lumps, and then express some of the milk. This may help to alleviate your symptoms.
Duct blockage. If you have a very hard area on your breast it may be a duct that is blocked. This can happen if your breast has not emptied fully during/after a feed. Try a warm compress and some Ibuprofen can be helpful as well. Continue breastfeeding/pumping to allow for continued release of milk.