My mother used to swear a lot, and now she finds it offensive, especially when I do it. Any reasons behind this?

It must have took her time and effort to overcome her use of curse words.
It is the same as an addicted gambler,alcoholic etc.
When they struggle and finally quit,many actually hate others that
still have the addiction.They forget and deny they used to be that way.

I used to do it a lot. Now I hAte it.

It's embarrassing

why don't you ask her ? doh?

Maybe you don't swear with the right words at the right times, so it just sounds silly to her when you do it.

People can change. Has she discovered religion by any chance?