Please help!

So I'm two weeks late on my period and I've never been late before. I went to buy a test today but they were all in secure cases were only a member of staff could get them out!! I was a bit embarassed so I went with the cheaper one that wasn't in a case. I did the test which came out negative! But I'm scared as to why I haven't come on? Any advice would be grateful! Thankyou X