13 days late??

Helen
I haven't been this late since high school & the only reason I was late or skipped in HS was because I was really active in lacrosse.
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TLDR; I'm a junior in college & the latest I've ever been since then is 5 days. My last period was April 8-13.
Not on any pills or anything.
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I went on a "no-sugar" diet & lost 6lbs (which is a lot for me because I'm 5" & WAS 95, now 89, so I had a weight change. Not sure WHEN I started dropping weight though. Sometime around Spring sem.
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I took a break from work last sem & then started working again the 2nd week of May (8th) right after I was stressing about my finals (but I always stress before tests, so this isn't ABNORMAL), so I did a routine change from studying everyday & stressing to working.
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My boyfriend & I happened to have sex May 11th (week I came back to work). We unfortunately didn't use protection, but we both recall him not finishing inside me. 
No need to lecture me on how I should've because we're past that. 
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My obvious thoughts go to "I might be pregnant", but I took a test & it came out as negative? 
I'm taking another tomorrow though (too dark out now) just to make sure.
But my other thought is that the weight & routine change is also affecting it?
Could it affect it this drastically? 
I don't know guys. I'm just so freaked.