What Causes Spotting In The Mid Of Menstruation Cycle?

I had what can only be described as Implantation Spotting on the 18th of May. I wanted to find out what it was since I have 2 children and have never experienced it before, so I went to the ER, it was late and doctors were not open. The DR said he ran a pregnancy test but honestly he thought it was to soon to tell, that what I mentioned above was the best idea of what I was going through. I have been waiting and took a test to see if there was any indication of pregnancy and it was negative. I am now having some symptoms I feel are familiar, increased urination, soreness in the breasts, cravings and nausea. On top of that I just don't seem to be as upbeat as I normally am. I want to know if I should take a test in two weeks or if the DR was wrong. On the 18th, for about 4 days, I had pale pink spots on tissue only, some light brown and mucus. It was not constant and never heavy like a period. What is your opinion?