It's the hoses.............. PERIOD They do this all the time. Change them!!!
Hang on - since only one rear caliper is sticking, and a single hose serves both rear calipers, I wouldn't be so quick to blame a swollen hose. But, hoses are cheap, and your's is probably of unknown age, so replacing while things are apart it is probably prudent.
But, it could be a stuck piston as well. The od of the piston can get corroded or gummed with dried fluid, causing it to hang up on the square cut seals. Since putting in new pads, you are exposing a different part of the piston to the seal. So it isn't surprising that this problem arose after the pads were changed. Cleaning/replacing the piston usually solves this.
Originally Posted by Antiacus
i did contact a local brake shop and he just said "replace the calipers, it'll cost less than having me rebuild them".
Yes, probably true. If you are paying a pro to do this job, using rebuilt calipers will save expensive labor. But as others have written, I'd be careful about trusting a chain like Midas with an Alfa. On the other hand, if you don't have an Alfa specialist there in Springfield Oregon, any independent mechanic experienced with German cars will be familiar with the ATE system installed on your Alfa.