Help Me I'm Late On My Period !

Nayeli
So I had protected sex on April 14,2016 my period came on the 15 normal like always for 7 days. Now I'm late what it looks like 4 days late and in always early but i don't know what's wrong! I took a pregnancy test and it came out negative. I also been working out and I'm in a track team and eating a lot of chips but I'm not sure what's wrong can someone please help me !