It is normal for a piercing to get irritated with jewelry change, no matter how old it is. When I change my piercings (some of which i've had for years) they still get red and a little "angry" that I messed with them.
When you put the jewelry back in, was it the same jewelry that you previously took out of it? If so, I cant see that the jewelry itself is the problem, otherwise you would have had problems before.
You said you left it out for a few days, right? When you put it back in, you might have had to break a few layers of skin inside of the piercing to get it back through. This could easily be the cause for the irritation. If it does not improve in a few days, i'd go to the doctor and have him check for infection to be on the safe side.
