I hope someone here can help me...

My fiance came to visit me (we don't live together while I study at college) beginning of december, and to be honest, although I definitly remember booking into the hotel with him, I can't remember anything else of that night. I also can't remember having my period a couple of weeks after, when it was due, although this didn't bother me at the time (i was under a lot of stress at home, with family and suchlike nonsense at christmas). Between about 23 december and now i've been off the pill, but recently started to feel tired all the time, wasnt' sleeping properly, my discharge has got a lot heavier, and most recently my breasts are extremely tender, and seem bigger (or at least more 'pert', I suppose). I felt queasy last night, and have had headaches more often.
Thinking this was becuase i'd come off the pill, I started taking it agian on friday (13th, of all days).
I've just been to the doctor, who suggested off-hand that I might be pregnant, and then dismissed it becuase I told her (and I don't know why I said it, reassuring myself, I guess) that I had had my last period at the right time.
Could I be pregnant? And will taking the pill do it any harm if I am? Please give my any advice you can, i'm so worried.