i am confused- you said your period should have started feb 15th? if thats so, you are not late. and yes, if you take a home pregnancy test it will still come back positive, even when you miscarry, it can take up to a few weeks for the hormones to go back to 0.
I am in a very similar situation to you. My husband and I are trying to concieve, I am 13 days late today had all the syptoms you mentioned and a few more. But today started having dark almost black clots, but just a few. Not sure if this is the start of my period and is black because I am so late, ( normally very regular) or if something else is going on.
Oh and my periods are normally very light, last just a few days, and are almost always right on time