I'm really worried, my period is 18 days late, however 3 weeks ago I had slight spotting which lasted 2 days, I've had stomach cramps, but not in the last 2 weeks, I've felt really sick and my boobs are tender, also had frequent urination, please help negative pregnancy tests but no period

Time to go see the gyno. Get a blood test and go from there.

It is possible to have a chemical pregnancy. That is when the egg is fertilized but it doesn't stick inside the womb so nothing develops but the body might still produce pregnancy hormones for a while preventing your period. It's probably best to go get checked out just to be sure.