How early can you catch on that you're pregnant? What are are signs and symptoms to look out for? Whats the earliest sign to notice?

My best friend told me she suspected that she was preggers a little after 2 weeks. Besides the missed period and a touch of nausea, she said that her breasts were oddly painful and not sure if it is related, but she swore that her nipples were darker in colour and seem slightly "bigger".