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Not sure what happened (would be easier to tell if you'd post some of the photos) - but yea, probably they just developed your slide film in C-41.
Some people develop their film in the wrong chemicals deliberately, then its called "cross processing" / "x-pro" - and this usually gives very strong contrast and a shift in colours; often blues turn to cyan very much etc...https://www.lomography.de/search/photos?order=popular&query=xpro
Yes, they need different kind of chemicals to be processed - the "normal" (colour negative) film's process is called C-41, the one for slide film (colour positive) is E-6.
Most labs still do both, some only do C-41. For you it makes no difference, you just give them your exposed rolls of film and some monies, they develop it (and send you scans if you like).