What did I step on to cause rapid swelling and muscle pain?

I was walking in a wooded area and got stabbed in the ankle with a twig with thorns that looked like it came from surrounding trees. Within seconds it swelled and became very painful. The muscle, tissue or whatever it is is hard and throbbing. This happened a few hours ago and the pain nor swelling have decreased. What did I step on, what makes this happen, and what should I do? THANKS FOR ANY HELP!