Been trying for months

Ok so me and my fiance have been trying to conceive for months now with no luck. I have never missed a period or been late on a period since we've started trying. I have a 3 year old son so I feel like its time to try for another. My period is now 5 days late with no spotting or any cramping and I've tooken 4 test and all have said negative. Maybe its to early to test or what? Anybody else ever experienced this where one week it was negative and the next was positive?