Is “bitch” a dirty word?

Hi. I’m new. Like a lot of you, I’ve learned a lot about feminism in the last few years and have slowly but surely learned to recognize that many of the things I used to be okay with, simply can’t be reconciled with calling myself a feminist.
A lot of people use the word “bitch” in their daily vocabulary, especially when describing a woman they feel is rude, perhaps disrespectful, perhaps snotty or even bigoted. We use the word “bitch” as an adjective as well, saying something like, “That job was a bitch.” But the term, whatever part of speech it is, is always derogatory.
We’ve accepted in this movement that the words “lame,” “gay,” and “retarded” (among others) are not appropriate. We go out of our way to explain to others why they shouldn’t use the word “gay” to describe something or someone they don’t like or that is unpleasant. Can the same thing be said of the word “bitch”?
The word originally meant “female dog” and apparently (according to Wikipedia — I’m not going to do a lot of research right now) has been around in the derogatory form since 1400. It also is used to degrade men (i.e. “He’s my bitch” in prison and “son of a bitch.”)
Recently, however, the feminist movement has re-appropriated the term, similar to how the fat acceptance movement has reclaimed the word “fat.”
But as those of us in the FA movement know, while you may be comfortable calling yourself fat, you probably aren’t comfortable using the term to call someone else fat, regardless of their BMI.
Which brings up the question: When is it okay to use the word “bitch”? What do other feminists think about this? Is it okay to call yourself a bitch, but not okay to call other men or women it? Should you say something when someone describes a woman as a bitch in the same way that I would say something when someone uses the term “gay” in a derogatory way?
In other words, is the word “bitch” a dirty word?