If someone calls a woman a bitch, they are saying in a very rude way that they think she behaves in a very unpleasant way.

If you describe a situation as a bitch, you mean that it is very unpleasant or difficult to deal with.

If you say that someone is bitching about something, you mean that you disapprove of the fact that they are complaining about it in an unpleasant way.

A bitch is a female dog.