My period came a week late, and it's a lot lighter than usual. I also got a big blood clot, 2 in wide and 1 in thick, could I have been pregnant?

Maybe not, but... Pregnancy is the most common cause of a missed period. If not pregnant, other causes of a late or missed period include: excessive weight loss or weight gain, increased exercising, emotional stress, having an illness, traveling, medicines (especially birth control pills), hormonal disorders, etc… a visit to one’s ob-gyn doctor is a good idea if symptoms are persistent, recurring, or worrisome.