i too am going through the same thing. I had my period a lil over a week ago. The last day of it was exactly one week ago. Two days ago I started spotting, it wasn't brown or abnormal in color at all, it was the color it would be during a period, though i had already had one. And the spotting is light, and its off and on. The 1st day it lasted on an hr or two, perhaps 3hrs at the longest. The 2nd day it was half the day. And today, being the 3rd day of spotting, its been light spotting all day long. I took a pregnancy test, and it was negative. Along with the spotting, the inside of my vagina has been hurting, sorta a throbing feeling. If you have learned anything or might know, let me know.
Also, about 5mths ago i got diagnosed and treated for P.I.D. after the meds were all taken that were giving for it, i felt better. But since then, every period i've had has been totally abnormal. Not sure if its associated, but figured worth mentioning. This may help. Try going the the ER during the day, thats what i did when i found out about P.I.D. a few wks prior to the ER visit, i saw my doctor, they ran lots of tests, and said everything was fine. So its possible your doctors could have missed something.

i had unprotected sex came on my period a week early and now that im off i still was spotting i no my body well once im off im off i don spot extra days and wen i read about it it comes down to pregnacy (implantion bleedig)... and ideas of what tis mite be