My cat is suddenly aggressive towards my dog

Hello all, I'm a 20-year-old from the states and 3 year old my cat Shady has recently gotten very aggressive towards my dog. For some context, she was here before our dog arrived, so she has never been a big fan of him but has typically been able to handle at least being in the same vicinity as him. But in the last 2 days she has started attacking him for even being remotely near her. She does have kittens, they are a little over a month old now, and this isn't her first litter. But this has never happened during her previous litters. It first happened the other night, my mom came in the room for whatever reason and our dog followed her and we thought maybe one of the kittens hissed at the dog and Shady just went into overdrive. But today she and all the kittens were hanging out in the living room with me when i heard my stepdad come home with the dog so I started getting all the cats into my room since that's where all their stuff is. Shady stayed behind, which I'm pretty sure is due to one kitten having stayed behind, and without hesitation started at the dog once he was inside. I've got her and all the kittens in my room now and have boarded up the cat door i have on my room door as she keeps trying to go out there. I'm not sure what to do, I would bring her to the vet, but I don't have extra cash to bring her just to find out nothing is physically wrong with her. Any help and advice would be very much appreciated.