abit worried, help please

I've missed my period. It should of came on he 21st April. Haven't taken a pregnancy test yet. But I've had no symptoms. Yesterday I noticed that only my left nipple is sore when I touch it. I'm worried because it's only my left one. Has anyone had anything similar and what was it because of? I'm going to take a pregnancy test tomorrow but I'm just worried because it's only my left nipple. And I'm 21 if that helps. Someone help please