Emma, it could be a few things. One thing that springs to mind is pyoderma, a skin infection that can cause crusty lumps, that feel like boils under the skin. My Indie had two of thse undr his armpit ages ago, and I freaked out too. I was surprised to learn it was a pretty minor skin infection.
Also, she could have been spiked by a twig in the park, and have a we abscess. Phone the vet first thing and let us know how you get on, I know how scary it is. xxxx