Is this a disorder and what would it be called?

Over the past year to a year and a half I've begun to mix up words in speech excessively. It's like I'll have the sentence formed perfectly when in my head, but, as soon as I try to speak it, I mix up two words when comparing things and say the exact opposite of what I meant. Sometimes, I'll also mix up single words like I'll say "roof" instead of "ceiling" or "geometry" instead of "geography." I'm beginning to get concerned about this; I'll look like an idiot half the time and will try to just pass it off as "you know what I mean" then have it happen again in another minute of conversation. It's happening purely with speech; I have no problems with text or written words besides forgetting how to spell long words sometimes. What is this?