I've been thinking about this - It may be that he is sucking for ages because he wants to be near you - a separation anxiety stage rather than an actual food-related thing.
My DS will flap at me when he is really tired - maybe try feeding a little bit earlier?
The reasons for low milk supply at this point that I can think of are: less feeding (i.e. after he's been having a few days of feeding frenzy it will fix itself); pregnancy (so he's going to have to get used to it!); return of period or taking sudafed (which should fix themselves in a few days).

Also - he may not want to feed lying down any more - is he any better sitting beside you, or with you lying on your back and him on top??

good luck
Kate