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?

I noticed around 3-4 weeks pregnant! One morning I woke up to a moment of sharp pain (something like menstrual cramps). I took a look in the calendar and got suspicious. The cramps continued for a couple more days but it was still too early to tell for sure! My period was getting late but the pregnancy test was negative so I had to wait and repeat a couple days later. When it turned out positive, I wasn't even surprised ^_^"