am I pregnant ?

Me and my boyfriend were having sex around two weeks ago 
It was very intense at the time and he wanted to cum in me but I'm not on birth control so he decided to do do it in my ass (sorry if this disturbs anyone) I let him do so. I am now three days late for my period, taken a pregnancy test that has come back negative. I am very rarely late for my period... Is there a possibility I could be pregnant ? 
I'm aware that cum must go into the vagina to be pregnant but could this be possible ( I know this sounds a bit dumb but im a little scared) 

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