My boyfrind lied to me about something stupid, why?

Last night he texted me "Its your birthday! :D" Then a few seconds later "Ooops sorry <3". So I asked whose birthday it was and he said Kyles. And I had this gut feeling like he was lying for some reason.. So I clicked on Kyles facebook and it says his Birthdays in July. I dont understand why he'd lie.. Is it because I get all quiet when I know hes texting a girl? But I mean hes telling somebody happy birthday, why would I care. Ughhhhhhhhhh WTF?