Hair Loss with grain like sand on my scalp?

[redwing3]

I have been recently losing my hair.   I went to a Derm she said i had male pattern baldness and put me on propecia which I have been on for about 15 days.  Well I still seem to be shedding hairs like crazy.  I also have been under in extreme stress when my gf of 4 yrs broke up with me 3 months ago (so not as much stress now but at the time it was intense).  I have been shedding hairs ever since.  To make matters worse I had blood tests done and everything came back normal except I had low iron in my blood.  So now I have been taking a multivitamin with iron for a couple weeks.  

But what I dont understand is that I still seem to be shedding a lot with little white bulbs at the end of my hair.  When the hair seems to shed it is also of very different lengths. Some long, some tiny.  Also if I scratch my head 5 or more hairs will fall out.  Along with the scratching i notice that i can pick up tiny white/yellowish specs, like that of a grain of sand on my scalp.  Any ideas to what these little specs are and if they could be contributing to my hair loss?  Is it normal/good to lose hair with the little white bulb at the end? Lastly why am i losing hair at different lengths?