Period is two weeks late.. and now spotting?

We have been TTC for 2 years. I do have PCOS and I keep it controlled with vitamins, diet and moderate exercise. For the last year my periods have been regular and I have not changed anything in my routine. My husband and I decided to do an IUI this March and made the appointment, so I had been sitting around just waiting for my period to get here.

One week late, I took a home pregnancy test. It came out negative. I have had no physical signs of my period coming at all aside from light occasional cramping and fatigue. I'm experiencing a few other symptoms but haven't blown them out of perspective just yet since we've been trying for so long... Today I went to the bathroom and after wiping, I noticed that in my discharge there was a smear of reddish brown. I wiped again and got a small spot of it. I wiped once more and it was gone and I didn't have anything else since. My husband and I have not had sex since the day my period was due. My period will be two weeks late on Wednesday.

I spoke to my RE and he wants me to wait until I'm three weeks late before going in for a blood test. With my first pregnancy, I had three negative pregnancy tests and one error. I did not get a positive result until my 8th week. I didn't experience any spotting of any kind before. I know that I ovulated.

Any thoughts on this? What would cause the bleeding? I've been told implantation bleeding, but that wouldn't occur almost two weeks after my period is due, would it?