Late w/ negative test results.

Okay so here's the deal, I have three kids already so I'd like to think that IF I am pregnant I would have some sort of idea. Now knowing that all pregnancies are different I need some advice. I usually have pretty regular periods. If I am late it's only like 2-3 days. I am now 12 days LATE. My Husband and I don't mind having another baby, we don't use birth control and we BD a few times last month. The reason I'm posting is because I have taken 3 pregnancy test and all of them came back negative the last one was yesterday so 10 day late. I did how ever have a streak of blood one day when I wiped. It was just one streak kinda like a piece of red string, that was it. That was like 4-5 days ago. Sorry TMI. I'm confused as to what may be going on. No added stress in my life. No weight. No weight loss ( boo, LOL ). No new excercise/diet. And I'm only 26 years old. So should I wait till like next month before I freak out or should I just take another test this month sometime? I would sched. an appt. w/ my OBGYN but either way it'll take a few weeks before I can get an appointment so... what do you think?