Assuming pads are same type. Sounds like you did not bleed/flush the brake fluid when you changed your pads. Sometimes you can get away with not bleeding, but what can happen is old, gunky fluid will be in your caliper. When you force it back into the bore to fit the new pads, that gunk is now re-introduced to the lines & reservoir. This is bad, mmkay? When pushing teh piston back into the bore, crackopen the bleeder nipples on teh caliper. That grunge can result in a sticky caliper.
Also, have you ever flushed your brake fluid? Over time it goes bad (absorbs moisture) and needs to be replaced. IIRC FSM says 2 yrs. 4 yrs is Absolutely longest you should ever go, assuming it doesn't sit in a garage for 90% of its life, like a show car.
I'd wager you have a fluid or sticky caliper issue. Flush teh fluid, clean & grease the caliper pins very well, and double check there are no tears or in the piston skirt too