It's around this point that your supply often starts to regulate and you're no longer as engorged and leaky as you may have been -- this also often means that you won't be able to express as much as you could in the early weeks. Please don't judge anything by what you can express. If he feeds at the breast often enough then your supply will almost certainly adjust (if it needs to) to meet his needs. Someone on here once described frequent nursing as like leaving out an "extra pints, please" note for the milkman.
If you are worried about your supply, feeding lots and getting lots of skin-to-skin contact is the best thing. If you can, taking your baby to bed for lots of skin-to-skin contact and feeding for 24 or 48 hours is an excellent idea. Also, make sure you always offer at least two sides, or even three or four (i.e. offer side A, then side B, then side A again).
That brings us on to your painful nipples (because feeding that much can take a toll on them if you don't have a good latch going). I strongly suggest you try to see a breastfeeding counsellor to check your latch, if your nipples are in that state. Using nipple shields as a short term measure can be invaluable in some cases, but used long-term they can cause problems. Again, a breastfeeding counsellor could give you advice and support in using them and then gradually stopping using them again.
I breastfed DS for 3 years and have been exclusively breastfeeding DD for 26 weeks, but I've never managed to express more than about 30ml even at my most leaky (and it would normally be about 15ml). It really isn't a guide.