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During the replacement, one pad was hardly fit into the slot and I had to push very hard to get it in. I am not sure whether this could cause the problem.
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You may have nailed it on the head with your comment about the tightness of the pads. The pads should slide into their metal keepers, it may take some wiggling or even light force but you should not have to force them in, that is incorrect. Brake squeal is almost always caused by pad vibration when the brakes are applied, the pads compress against the rotor when you press the pedal of course, but what pushes them back out is only disc runout (the disc brake pads on most cars have no return springs designed in, nor do they need them).
So you know that the pads must be free to slide inside the caliper.
As for what to do about it, if you own a metal file you can simply file a few strokes of the metal backing portion, at the point where it appears to be sticking. The backing material is only mild steel and files readily. (Dont use a grinder, you will take off too much metal - use a file!)
How to do this, remove the metal keepers and fit each end of the pad in. Then you can see where the metal part is binding and file it a bit. replace keepers and install the pads.
also to prevent squealing:
-did the pads have shims when you removed the old ones? These shims need to be reinstalled, or replaced with new ones.
-the blue chemical anti-squeal (the sticky stuff) should be applied to the BACK of the pads. Often new brake pads come with a small package of it if not, simply buy some. Apply to back of pads - keep it off the lining. Allow to dry, reinstall pads.
-are your rotors new or turned? If the old rotors are still smooth, it's possible to simply put new pads over old rotors, and i have done that many times with no problems. However, a good idea is to sand the rotors lightly with fine or very fine emery clothe. (The purpose of doing this is to break the glaze from the old pads.)
Simply make one to two passes sanding both side of the rotor, going across the rotor surface once then round and round once, then clean it well with brake cleaner, and this will eliminate the glaze. (If the rotor needs more attention then this, change the rotor or have it turned.)
-make sure the sliding pins in the caliper (the ones tucked inside the rubber boots) are clean and lubricated with high temp brake grease.
as to whether you should lube the sliding parts of the caliper or not, it may be a moot point because any grease simply attracts brake dust, thus cancelling out the lubrication. Often though i do use a small amount of lube on the metal part of the pads (where they go in the metal keepers) being sure to keep any lube off the pads.
I'm not sure whether this has much effect or not because every time i come back 30,000 or so miles later to change the pads again, the lube is not just dirty, it is completely gone..so put the pads back together dry or lube lightly, whatever you think, probably doesn't matter a whole lot.
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