a little confused... help

Okay so I've been tracking my fertile/ ovulation days to prevent pregnancy. My last period was on March 11th and I had unprotected sex on March 25th (ovulation day). The next day I took a morning after pill. My next expected period was due for April 7th but instead it can March 31st which was strange because it was a week early and it only last for about 2 and a half days. Normally my periods are 4 days long. I spotted on April 20th and haven't had another period since. Is there a chance I could be pregnant? Does Plan B throw off your period? I took a test but I came back negative. Help! Do you guys see a faint line?