I am going through the EXACT same thing. I'm on cycle day 18 though..... three days ago, I got a very, very dark second line... I tested the same day, later on that night and it was clearly negative--barely a second line... and it's been like that for the last day or so... I tested again today around noon and it was very, very dark again. I think I ovulated yesterday or the day before because I've been having EWCM... my had a temp rise this morning as well--- but I'm not sure how to read my chart yet..
So, I really don't know what's going on with me either... I took a digital hpt just in case.. and it was negative... so, I hope someone can answer this for us both.... hang in there!