Why is my period 12 days late???

Ok so let me start of by saying I AM still breastfeeding my one year old (13 and a half months old). And this caused me to have one period between the time she was born and when. She was 8 months old. Since she was 8 months though they have been regular. I have been getting regular periods for 5 months now up until this last one. I have taken quite a few pregnancy tests and they are all negative except the last one my best friend SWORE she saw a second line and so did her husband but to me it was far to faint to count. Like so faint that my phone camera wouldn't pick it up. That was Thursday that I took the test. Yesterday basically. I feel like even at that point bring 10 days late it would be a clear positive if I was pregnant. I just don't wanna go crazy and spend a ton of money on more tests when it could be nothing. And I know I know I should see a doctor. I probably will of I can get a straight answer soon but my work schedule doesn't allow much of time to go to the doctor if I'm not actually sick. Any one find out they were pregnant with several negatives after their missed period? Could it be breastfeeding making it irregular even though it's been regular the past few months because of breastfeeding?