You can just jokingly say, "Hey, I'm having a baby here! What do you think, I'm made of money? All these coffees and lunches are taking diapers off her bottom!"
Or maybe you can volunteer to MAKE you both lunch from whatever's in her kitchen, telling her you really can't afford any extra expenses with the baby coming so soon.
Or just call her out on needing to pitch in, walk in with whatever you bought and tell her it was xx total, so 1/2 would be...
If you'd rather go the passive/aggressive route (which we women often like
) how about if you "forget" to pick up whatever you said you would on the way over? Get there and go "aw, jeez, I had a pregnant moment and totally forgot! And oh, look, I'm out of cash and don't have my debit card. Do you still want something? If you buy, I'll fly..." and see how she reacts. Then you can make a point out of paying her back next time you see her. My guess is that she's also distracted with the new baby and not focusing on anyone else right now; she probably also can't just run out to get a coffee or something to eat easily anymore and misses it.