You can definitely be pregnant for 2-3 months and not know.
I didn't find out I was pregnant until I was 6 months along...I know that sounds ridiculous, lol, but this is why:
~I had been on birth control pills since the day I turned 18, so it had been 2ish years by then of "control".
~I was still bleeding regularly every month at the same time, for the same amount of time...not heavier, or lighter, or longer or shorter.
~I wasn't any moodier than I ever had been.
~I had no morning sickness (other than a bout of supposed appendicitis that wasn't really appendicitis, but a near-miscarriage...that was maybe 3 months in, but the doctors didn't check for anything other than appendix problems. They just gave me pain killers and fever reducers and I was better in a week).
~I was very active at the time. We went bike riding every day with a friend, for hours at a time, and it was summer so it was 90ish degrees every day. We also went on walks and the occasional jog, so I barely gained any weight (I was already underweight at the time, so I was supposed to eat more to gain weight but we were exercising to stay in shape at the same time).
Point being, don't ever scoff that someone didn't know they weren't pregnant for any amount of time. It happens! Sometimes your body may fool you into thinking you're not pregnant, but it -can- happen.