I can't say whether or not it's normal, I can say that through two pregnancies I have never EVER had it happen and can't recall speaking to anyone who has had it happen. I would call your doctor and see what they say!
It could be nothing or it could be a sign of a problem. The only way to tell is to call your dr. and get checked. I have heard of people who had bleeding all through their pregnancy and were fine but others have had slight spotting and it was a sign of a problem. Get it checked out.
I had a trace of blood in my urine a couple of times during my second pregnancy. It my case it meant absolutely nothing. I asked several doctors and nurses, and they all told me it was normal, and I had nothing to worry about.