I got my left nip pierced yesterday....?

Well, since you just got it and pierced a hole through it, your body will react to that causing it to temporarily swell up. Nipples do not constantly grow. Whatever size they were when you got it pierced is the size it'll go back to; it will just have a piece of metal or whatever going through it. Your piercing will be sensitive and tender for a few months minimum, but it will heal up given time. I have several piercings and have experienced this many times.