Progesterone and... Spotting?

My doc prescribed me progesterone thinking it'll be the trick we need to get pregnant.  My cycle has always been incredibly normal - 26-28 days.  Zero spotting, ever.  Until this month when I used the progesterone for the first time... I started spotting on CD 18!  Now, CD 23 I finally took a test (negative) and have what I think is the real flow.  Please help, we are going on four years of trying.  He's fine, I'm fine- completely unexplained and we have amazing and healthy five year old.  Thank you in advance,