I need advice.

I also need to clear my head a little bit.
I'm worried.
My period is three weeks late. This is scaring the crap out of me, I don't really have anyone I can talk to beside my boyfriend and he is as clueless as me. I haven't been this late since we've started doing stuff.
I've done some home pregnancy tests but they said negative. I don't know how reliable they were so I don't feel any better for it.
I can't go back to the doctors because I go back to work again tonight so by the time they open I'm asleep.
That's another problem, I'm exhausted all the time. I honestly don't know if I can cope with my job much longer. It's killing me and, if I am pregnant,the stress they put me under would be so bad for the baby.
My boobs are also killing me and the other night we were walking to get the pregnancy test and my head was spinning. I think that might be stress though.
I just don't know what to do any more.