How common is a false negative on a home pregnancy test?

Ok so first off I have a little thing called PCOS. My husband and I have been trying to have a child for a while now which is how we discovered that I had PCOS in the first place. I have taken sooo many hpt's with a negative result that I'm very discouraged about taking them again. I don't have a very regular period but since starting the Metformin the past three months it has been like clockwork until this month. My husband and I celebrated Valentine's day like most couple's probably did and then about 5 to 6 days later I started bleeding..not heavy by any means in fact it was very very light it definatly did not resemble Aunt Flo and it only lasted about 3 days when normally my periods last a full 5 with heavy bleeding. So at first I was discouraged thinking that my period was off course or being weird. Then I started cramping and having lower back pains I started thinking that maybe my period would start after all, only it never did. I also had some severe mood swings in which I would get really sad for absolutely no reason. I was supposed to start my period on March 4th and it is now March 16th I took a hpt the day I missed my period then another one a week after, all showed negative results. I still have cramps off and on. I never really paid close attention to the area around my before but it seems like its darker and they also feel sensitive but not really painful..I guess I'm just wondering if its common to get a false negative and if I should maybe have the doctor run a blood test?