I've noticed that after a fiesty fight with my girlfriend, we both feel a sence of relief and happiness. Sex gets better. Why is that?

All things come out! Literally! Watch how feisty, free to argue, but you need to listen to each other. I've learned that listening makes my job and relationships easier than blabbing and fighting to be heard. Respect and patience, that's how I make a living. No judging!
EMOTIONAL INTENSITY. Fighting and sex have one common brain neurochemical at play called dopamine. This same hormone is reponsible for many emotional responses such as euphoria, orgasm, anger, rage, happiness, drug addiction, and all pleasure type responses. Although there is no clear answer the reality is that the emotional intensity associated with arguing and sex may amplify each other outcome via orgasm.
How We're Wired. I suspect that this happens b/c when in "fight mode" our bodies start preparing us 4 danger - preparing us to fight or run very fast. In the process, our senses become heightened, we get filled with adrenalin, our hearts race, etc. This is exactly what happens when we get sexually aroused. So, not surprisingly, fighting can essentially feel like foreplay.