Help....not a clue!!!

Okay....this is the weirdest month for me. My last period was 1/23/12. I have an approxiamately 28-29 day cycle. I do not chart. I am stumped right now though because......

for the last 3 days I have had cramping. I had some pretty bad cramping on one side on the 18th. I have tested everyday since then with stark white negative pregnancy test. AF is due today and I have absolutely no symptoms of her coming. But no real symptoms of a pregnancy either than the cramping. I NEVER cramp prior to a period, and I barely cramp during a period so this is very much out of sorts for me.

My question is do you think that the 18th could have been implantation cramping???? And if it was, about how long before hcG shows up in the urine? I hate wasting money on the tests, but I do need to know early for some other reasons.