4 days late but bfn's!?

I've been taking tests since 3 days before AF was due and they've all been negative. Except one, there was a slight line but it went away? Totally weird. I've been feeling off the last week. Gassy, tired, crampy in my ovaries, constipated, creamy white cm, ect. Not usual symptoms of AF. What is going on? Has anyone else had something like this happen, then get a bfp? How long did it take? My 1st & 2nd pregnancy both tested positive on the day it was due!