Am i using a home pregnancy test too early?

my periods are pretty irrigular. my last period started around the 11th of december and finished on the 16th. i had unprotected on the 18th (well he pulled out but i know that doesn't really make a difference), anyway, i worked out my period should have been due around about yesterday. i've been getting the usual dodgy stomach that i normally get before my period starts but apart from that, nothing. yesterday, i used a home pregnancy test from superdrug which says it gives you a result 4 days before a missed period, it was negative. is it likely that my periods are just late or could i still be pregnant and i'm just testing too early? i've read on some internet sites that it's ok to test a fortnight after unprotected - it's been 24 days since i had it, but then other places say you should wait longer. any help would be appreiated, thanks.