A hard lump that hurts is a sign of a clogged milk duct; chills, body ache and a fever (if you have one) are signs of mastitis. I would go to your dr, because you may need antibiotics!
There are a few things you can do...
- feed in different positions; try alternating cradle hold and side lying, with football thrown in sometimes.
- gently massage the area(s) when you are in a warm bath or shower as if you are pushing milk out towards your nipple
- no underwire in your bra, or at least not every day; don't sleep on your tummy
- make sure that she is emptying at least one breast entirely with each feed and make sure that you are feeding often enough (every 2.5-3 hrs at her age)
- make sure you are getting enough rest (difficult with a newborn and toddler, I know!!!)
I have never used a nipple shield, so I am not sure about that one... sorry! Why are you using one, if her latch is ok? I would think that because it is something coming between your nipple and her mouth, it might be interferring with how she empties the breast. If the flow is too strong for her, try expressing a wee bit before nursing so that you can put her on after that flood has passed. You can save the expressed milk to feed her, if you want.