First, breast size changes and engorgement don't mean much. Some women's breasts do that and others don't. If you have larger breasts, especially, it tends not to happen. If you have smaller breasts, it still doesn't mean much.
Also... Are you EPing right now or are you nursing in addition to the pumping? 'Cause lemme tell ya... I could never get ANYTHING out of my boobs when DD was nursing all day. Not hardly a DROP. The only time I could get anything was when she hadn't nursed for 3 hours or more. And even then, I'd likely not get much with my pump, especially before I learned to use it properly. Check out this link
for more effective pumping methods. It's not as simple as you'd think.
I'm going to third the lactation consultant suggestion. Also, see if you can rent a hospital-grade pump for a month or two or even just for a week. See if you can pump more/better by using that. If you can, it's not a supply/IGT issue so much as a supply/demand issue (i.e. that you need better milk-removal protocols, such as a better-latched on baby or better pump or both).
IGT is pretty uncommon, but if you have a real true reason to think you have IGT (like, mother or sister has it diagnosed or your lactation consultant says so or something) then you might consider taking Goat's Rue. It's used a lot of times to help women who've had breast surgery build up to a full supply and works by increasing glandular tissue. It's also commonly sold as a "breast enhancer". (to make your boobs bigger). Well, you could consider taking it anyhow, though. Also, if you're not taking enough Fenugreek to smell like maple syrup, you're not taking enough to affect your supply according to Dr. Jack Newman.
Lots of women start off their first BFing relationship just like you have and end up believing that they can't make enough milk. Don't take yourself down that path just yet, ok? Give yourself the benefit of GOOD, SUPPORTIVE help. NOT mediocre help.
And don't feel like there's something wrong with you because you don't "get it" all at once and it didn't just magically happen. Breastfeeding successfully is a learned activity
. It's not
instinct and it's not hormonal knowledge. It's like learning to swim. It takes training and opportunity, not just getting thrown in the deep end without a life jacket.