As the new dragonborn DLC came out recently I started a new character, I'm around level 50 with a non-mage character, I haven't even touched any of the non main story quests for the college yet some how I have frost and flame thrall... Does dragonborn or Dawngaurd supply a second way of getting these spells without doing the rituals that I may have skipped? Or have any of you encountered this?

In Dragonborn you can find those spells in Apocrypha, you might've picked them up and accidentally learned them thinking they were books?