Maybe you are not pregnant since you already had your period for 3 days. Your shorter cycle and the symptoms you are feeling now could have been triggered by stress or maybe some sort of hormonal imbalance. But of course it's still important for you to see your doctor just to make sure. There are a few occurrences wherein the menses would occur and find out later on that she is pregnant.
You could still be pregnant. It runs in my family to have implantation bleeds. That's where you bleed because you are pregnant and it's a long explanation. It's always safe to check. That's why they always ask for your last NORMAL menstrual period.
Not necessarily. If you are on a low dosage Birth Control pill such as ortho tricyclen lo, it is very likely that your period will be shortened or might not happen at all. Take a pregnancy test just in case, but you probably have nothing to worry about if you are practicing safe sex.
As someone else said, the question is "your last "normal" period." I have been pregnant three times (and have 3 wonderful children). I had no bleeding with the first two but bled for 9 days with the third; it was 'different' than a normal period. Turns out it was implantation bleeding.
Yes, you can be pregnant even if you have a short or light period. I had a period that lasted 7 days but was extremely light, I was pregnant....8 months later I gave birth.