I think it is possible to have symptoms before your period - it's just that most people don't recognise them. I know when I was pregnant with my first child, I had sore boobs for two weeks before my period was due. As I didn't know I was pregnant, I was starting to worry about cancer! #-o
With my second pregnancy (again, I was clueless), I was extremely tired for two weeks before I missed my period. But the third time was planned, and I noticed a lot of symptoms! The first was the loss of a yellow mucus plug (sorry, gross, but quite a few of my pregnant bulletin buddies had the same symptom); then the tiredness started followed by flu symptoms. It was so frustrating that early pregnancy symptoms imitated the flu! I spent 12 days wondering 'am I pregnant or do I just have the flu?'. However, I remember the strongest symptoms I had were as follows: food aversion (which could have just been the power of suggestion, I suppose), heartburn (which I never get except when I am pregnant, so that was exciting!), blood nose (again, only get when I'm pregnant), and extreme tiredness. Hope this is helpful.

Sue