I have had a MRSA infection inside of my nose, your nose is the most sensitive part, I had to see a ear, nose, thorat doctor for, mine, they actually went inside my nose to drain the sore, I was also under the care of an infectious doctor, but if were me, just because I know how nasty MRSA is, I wouldn't wait, I would go to my doctor and tell him what I was experincing.
Probably becuase it is irritated. Keep an eye on your nose and make sure that after your full course of antibiotics, it is better. If it is not, then go back to the doctors as soon as you can.