I feel a little guilty - should I apologize?

That does not sound bitchy to me. I'm glad you addressed the situation.

I think you're fine and I wouldn't apologize. If you do I think it will possibly make her think you were in the wrong and she will go back to interrupting. I have no problem telling people I'm not finished talking on calls or meetings and I don't think it's rude. I'm just letting them know that I'm not finished.

Honestly, I would apologize. Not for what You said but the choice of venue. You obviously have had an issue with it for a while, it should have been addressed in private.

This is my line of thought too.  

Yes. Say something along the lines of “I shouldn’t have called you on the interrupting in front of another, but I would appreciate it if you would dial it down.”

The "why do you always interrupt me" part said in the presence of another coworker is a bit harsh. Unlike, please let me finish or please don't interrupt, it implies a constant annoyance. I'd apologize.

I think “Excuse me, I’m not done speaking,” or “May I finish?” or similar is to the point and appropriate. “Why do you always” crosses a line if someone else is part of the conversation. Actually, “why do you always” or “you always” or “you never” can make people feel defensive and possibly hurt even if it’s just two people talking. I’d briefly apologize.

As part of the larger conversation, I might say it’s difficult to complete a thought/make the point when being interrupted. (That’s assuming the person is a known interrupter and I’m not going on too long, which doesn’t sound like it at all in this case.)

i’ve had to work on not interrupting, which can stem from being really into the conversation, IOW agreeing, not disagreeing. It’s still interrupting, so not good. There can be more than one reason, from social awkwardness/wanting to fit in to proving people wrong to wanting center stage (thinking of a previous co-worker, ugh) but sometimes knowing what’s behind it can help deal with it. Or not.

I would apologize

Someone needed to point it out and constantly interrupting co-workers is much ruder than what you said. Don't apologize. Everyone else was probably glad that you said something.

I would not exactly apologize. Saying that you weren't finished was appropriate. Continuing with the "why do you always interrupt me" in front of others was less so. I would talk to her and apologize for saying that latter part in front of others, and say you should have talked to her 1:1 about this ongoing issue. Then explore how you two can work together more effectively in the future. But do *not* apologize for "taking your time back" in the meeting.

She should apologize for her interrupting. Let it go.

Don’t apologize if you’re not truly sorry. Amd in your case, I wouldn’t be.

You know what your tone of voice was and whether it was bitchy or not. 

A third party knows whether you were bitchy or not.

IMHO, it really depends on what sort of reputation you want in your workplace and relationship with co-workers going forward. Apologizing isn't being weak. You aren't apologizing for calling her out for interrupting but for doing it in an unprofessional way.

I guess I might apologize for not talking to her privately about it. The fact that you snapped (justifiably!!) to me says that you probably should have addressed this earlier. Anyway, it needed to be said - maybe it will make her more thoughtful.