Using Surf and Waterfall as Cradily.

I was a Walrein, with Surf.
I got bored of being Walrein. So, I became Lileep, then Cradily, and as Cradily I can surf. The HM says I can not learn it, however. So I am assuming this is some sort of glitch that happens when you've already learned the move as another pokemon? I also have the ability to go up the waterfall, though, Dive does not pop-up any longer.
Please, feel free to contact me if you need more information about what had happened, or something.

-Zaira.