Potential Pregnancy?

Hello!
I am curious. Many a time I have feared being pregnant, and many a time have I spent weeks worrying. Although, I never have sex. Although, I am aware that you are able to fall pregnant through general foreplay as can 'pre cum/ejaculation', make you fall pregnant. Although, I am very careful and we do attempt to be as clean as possible. My reason for worrying comes mainly down to how fertile my mother is. My reason for worrying this month however, is due to last week, I had a day or two of very light brown discharge. But nothing since then. I think I am due on, I really ought to make a note, I know. I usually begin on the same day as my mother, however she started on Monday, and now, its Thursday and I haven't yet. Usually, I get brown spotting just before and after, however it has been atleast a week since and not a sign.

Until recently, my periods were extremely irregular. I find with less stress, they come on time. And possibly more sleep? And although recently I have been very happy. The beggining of last week threw everything apart for me, and since then I haven't been relaxed. Neither have I been well slept as I'm constantly working.

I have all the signs of my period, cramps, PMS (I think, I'm not too sure if thats down to the stress), increased wind and abdominal noises. All very usual for me before coming onto my period. So, what are the chances of me being pregnant? I can say the past month and a half, sexual activities have been rare, and when occured, very careful.

I guess, I just need to put my mind at ease. And of course, invest in a re-usable pregnancy test, calander and a form of contraception (eg: the pill)

I'd really appreciate your feed back, thankyou!

Oh, and another thing, which does make me doubt the possibility of being pregnant. I think, the last time my mother and I were on - she lapped over a few days longer than me. I'll need to ask her that again though, just to make sure I am correct!