Should I be worried?

So I'm 18 and I'm on birth control but this month I missed my pill acouple days in a row and my doctor said I should take my birth control again whenever my next period ends. Okay fine. Well me and my boyfriend have had sex acouple times this past month using a condom. I was supposed to get my period acouple days ago I figured but still have yet to get it. I usually get back aches and headaches but I haven't had any of those. Not sure if I should buy a pregnancy test to be sure or not? Need advice loves