My AF is 13 days late, is something wrong?

[LaBellaVita4]

I'm now 13-14 days late for my period, what could cause this? I'm so confused! I have taken a HPT which was negative. My cycles are usually really regular lasting about 28 days. I'm 20yrs old and get quite a bit of exercise in, I've been losing weight, not a whole lot, but noticeable. Could this be a reason? I'm scratching my head trying to figure this out, dr refuses to do blood work unless AF is late another week or two. Anyone have any experiences or input? It would be greatly appreciated!!! Thanks in advance!

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