help!!

So I'm 18 days late, I have had sex (not in my fertile window) with condom and yesterday I tested with clear blue digital and it came out negative so I carried on with my day, I exercised and when I got home and went to the bathroom I spotted... does anyone know what does this mean? 
I have to say that My cicles are really irregular... and I'm 18 years old!
Sorry if my English isn't very good! 

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