Newborns are sleepy creatures. She sounds like she is doing fine. My firstborn lost about 15% of her bodyweight by day 5, so don't fret. Your LO will probably start putting weight on over the next week.
Mastitis is horrid and worth reading up on (try www.kellymom.com so you know the symptoms if ever you get it), but it's true that your breasts can feel overfull, lumpy and sore at this stage.
Personally I suffered frequently from blocked ducts (and a few times from mastitis proper - which happens when an untreated blocked duct bursts and the milk enters your breast tissue. The body reacts by sending lots of white blood cells to fight the "foreign body" in the breast, and you can suffer flu- symptoms like a fever & whole body stiffness (and it can come on very rapidly, in the matter of an hour).
In order to avoid mastitis proper, if you get blocked ducts try to treat them, for example, using a cold compress between feeds and taking ibuprofen (to reduce the inflammation), using a warm moist compress just before feeds (to dilate the ducts prior to your baby helping you suck out the blockage), and feeding frequently from the affected breast first. You can often see blocked ducts, they appear as pinkish blotches on the breast tissue, and can feel like small hard lumps.
If you have a problem with oversupply, you could use a couple of cabbage leaves (see Kellymom for details) - I found they worked really well. Just don't use that remedy more than 2 in 24 hrs as it is quite effective at reducing supply and in the early days you should be aiming to establish a supply (however, I used it when my milk came in and I was very uncomfortably engorged, and am still feeding 9mths later).
Good luck to you, you are trying to give your baby the best start ever, and breastfeeding is not easy, however natural it is! I am sure many MNers will sympathise with you, and I hope you have the support and love around you to help you keep going with it in these early days. A few months down the line, you will be glad you persevered - it is convenient, free, good for baby's health, and a fantastic way to bond with your LO.
Gotta get to bed now, but hope this helps.