i had that too just last week. i was 5 days late on my cycle and did a pregnancy test but i was negative. i was under stress at that time though due to worrying too much that i might be pregnant. but on the 6th day, i got my period and i was like "THANK GOD!"=). sometimes you can reach upto 40 days late from your first period on your previous cycle. if your period still wont come on the second week, i suggest you see a healthcare provider.