If you were able to pump anything in only 5 minutes after a long nursing session I really doubt you have any supply issues. 1/2 oz in 5 minutes is actually pretty good.
The absolute best indicator of adequate supply is your LO's diapers. Six wet diapers and several poopy ones tells me you are doing fine.
Keep nursing and try not to worry. Put the pump and supplements away. Instead, work on perfecting your latch and try nursing while laying down so you can get some rest during those long nursing sessions.