I was eating out and fingering my gf, she kept pushing down harder and harder, and I eventually had my whole hand in up to the knuckle in the middle of my thumb or so(btw i have rather small hands for a guy) and I'm pretty sure I was hitting her g-spot, and I felt the entire area just inside her expand like a baloon blowing up, and she just went nuts, and there was brownish fluid everywhere. It almost looks like blood but its really thinned out. Is there a chance I did serious injury? She's normally sore after I do this, but the reddish brownish stuff worries me.

No, as long as the blood is not bright red you have not done her injury. But you might want to back off a bit. Your fingers might be scraping the inside of her vagina. You are probably causing her vagina to contract - and if her menstrual period was recent - she might have residual blood inside her.

So no - no harm - just be a bit more gentle.
Susan