Star Wars Episode VII: The Force Awakens - She did not even know the Force was real until like a day before she fought Kylo.

Technically, the force is kinda genetic. People are born force sensitive, and back before the legends cannon became, well, legend, there were characters that could use basic force powers without training. Granted, I don't think any of them learned in the course of like a day or two like Rey did, but movies always move faster than other forms of media.

Let's be honest, given her quick learning to use the force, her ace pilot skills, and her all-around quick thinking, there's a good chance that she's just really strong in the force… just like her granddaddy (probably).