Dog Foster/Rescue Volunteer

I work (volunteer) for a grass-roots companion animal foster-based rescue organization. I am fostering a female labrador mix who we have no history on - other than she was pregnant, running the streets, no one claimed her and due to being pregnant, she and her pups were in danger of death. She was extremely timid and actually got away from me the first night I had her - but we've since developed a really great bond. She had her pups 3 weeks ago this Wednesday - but has in the past couple of days, twice now - growled and nipped at them. I trim these babes nails, and just did so last night in fact - and an incident occurred just a couple of hours ago. I realize this is normal behavior when they start to wean the pups, but they aren't ready to start weaning for another 10 days or so and will still require mom's milk for the remaining 3-4 weeks. I'm thinking I should not leave her alone with the pups and may even try hand raising them with puppy formula - but I've seen some great responses on this forum and hoped someone may have an idea of why this is happening as well as the best actions to take to stop this risky behavior in mama. I know that many of you may say she needs to be seen by a vet - but we are very short of funds with the rescue right now and I will probably be told we cannot afford this. I would personally fund this, but lost my job recently - which is beneficial in the sense that I'm always home for these guys and know what's happening when it happens. No lectures about spaying/neutering - because that, immunizations, de-worming and micro-chipping all 6 of these guys IS always in the budget and will be completed as soon as is possible medically. Thank you for any help you can provide!