Was overdue 2days for my period and spotted,was gone the next day?

my LMP was 30/12/06.i am a very regular 28 cycle chick,give and take 1 day and my period lasts 4-5 days.i was ova due 3 days in feb and on the 2/02/07 i spotted but not enuf to full a tampon also the blood being brown and pink.blood was gone the nxt day.not liking periods at all i ddnt think nothing of it and just carried on my biz.come premenstral which i thort was....my wer very sore to touch.come end of feb i was so tired i could hardly get up.then in march noing my period was due yet again i got nothing.had a hpt and it was very much positive.my point is i am now on my 36week and 3rd child tho very confused as wen i went for my scan it was 4weeks out from my period.i do not no wen i concieved and yes my partner and i had unpro in jan and feb.could i have been pregnant b4 premenstrual wen i had sore ...things have changed so much since my last child.has any1 had this happen to them...i just wanted to no bout all these symtoms i had...thanx 2 any1 hu can coment