How long should I wait?

Jenna

My period is now 6 days late. I'm usually on time or 1 day late but never more than that. About a week after ovulation I started getting super gassy. Like uncontrollably gassy. A day or so later I sneezed so many times during that 1 day I thought I was having an allergic reaction to something. The day after my period was supposed to start I felt fine, started eating dinner, got about 2 bites in then couldn't eat anymore. I felt a mixture of 'too full' and 'too hungry' at the same time that was just really uncomfortable more than nauseous, but then that persisted through the next day too. I took a pregnancy test yesterday morning (5 days late) and it was negative. Today at work my lower back started hurting really bad, like one giant cramp and I started spotting. It was so little that it only showed up when I wiped and it's brown. I've read about implantation bleeding but only that it happens before the missed period. Could I be pregnant? If I am I'm only 4 or 5 weeks. How long should I wait to take another test since the first one was negative? I've never been pregnant before so I have no idea what to expect haha. Thank you!