Question...

So I have a very irregular period. Well for the past couple of months it's been pretty normal. I normally get it heavy for 7 days. Past 2 months I've gotten it for 4 days and very light. This month, my period is 13 days late according to eve and 17 days late according to my period calander. I took a test, because I have unprotected sex with my boyfriend, and it came out negative... but still no period or period symptoms. I don't know what's wrong with me, maybe the test was wrong. Any advice?