gonna say NAH here, leaning towards a soft Y-T-A. i personally can't stomach the idea of doing anything with a used pad but throwing it out, but you and your friend clearly have the rapport where that's not a weird thing to do, or it's an inside joke. but your other friend is totally within their rights to be grossed out by this- nobody wants to see someone else's used pad!

if the gift opening hadn't been public, if it had just been a joke between the two of you and your other friend had heard about it and called you disgusting, i'd say N-T-A, but telling someone they're disgusting for gift-wrapping their own used pad and showing it off... i find it hard to disagree with this. (i do think that your other friend's comment that it's weird to "waste a used pad when some people can't afford them" is really stupid, though. it's not like you were gonna donate that pad, anyway). don't lose any sleep over guilt, but in the future try to be understanding that other people have different senses of humor and limits for grossness than you do