I took Levonorgestrel . . HELP!

Ok so last month a little after Valentine's Day I engaged in sexual intercourse. We used a condom but midway he discovered a mishap. We aren't sure if the condom broke or if it slipped off. The next morning he went out and purchased what I guess was Plan B it didn't say plan b it was just one tablet and the box and information said levonorgestrel (the active ingredient in plan b) I felt fine and thought all was well. I have read that it may beineffective in women over 165 lbs (which I am) but . .A week later I experienced some spotting for about 3 days. I assumed that mean it took effect but I called the company and they said I shouldn't be alarmed and should still be expecting a period. But as of now my period is an entire week late. This has never happened before I'm usually very cautious and I've never taken plan b. I'm not sure what I should do. . I'm way to nervous to buy a test.