Rules for notifying co-authors about re-presenting a project at a conference
From what you've said this sounds like a honest mistake made in good faith, and if I was your supervisor I'd accept your apology and move on. At your stage it's to be expected that you're still learning some of the norms of the culture, and they're not always clear.

That said, I do get particular about authorship. Let me tell you a story from my previous career...

I was a young PhD researcher, working in collaboration with several other staff and the head of my research institute. Let's call him Toby.

I came to feel that Toby had a significant conflict of interest related to our work, one which was not being properly handled. I felt that my boss was trying to pressure me to accept methods and interpretations that weren't scientifically justifiable, but which just happened to lead to results that were favourable to our institute's funding. Another co-worker felt the same way.

We refused to accept these methods, and soon after we were both fired. While we were serving out our four weeks' notice, we learned that Toby had gone to an international conference and presented these results in a talk and poster, and that he had listed us both as co-authors in this work.

If he'd attempted to do this in a journal publication, we would've been asked for consent to be listed as authors (and would've declined). But because talks and posters don't have quite as rigorous processes, he was able to slip this through. We ended up having to contact the organising body and request that our co-author credits be removed from the published proceedings. 

(Toby was furious. Toby also had right of veto over any reference that anybody in the organisation might write for us as we looked for new jobs. It was not a very enjoyable time in my life.)

As my experience illustrates, crediting somebody as a co-author isn't automatically a positive. When my name appears on a paper, I share responsibility for the contents of that paper, so it's very important that I have the opportunity to give or withhold consent for that to happen.

Two important things about consent: it's conditional, and it can be revoked. If your supervisor only agreed to the first poster, then you didn't have her consent for the second. You might think it's almost certain that she'll agree to the second one, but you still have to ask.

(I know my friend will happily lend me $20 if I ask, but it's still not okay to borrow it from his wallet without asking!)

There are reasons why somebody might say "yes" to one request and "no" to another. It might be that they have issues with the conference or its sponsors, or that in the time between one request and another their thinking has evolved and they no longer want to endorse material that previously seemed okay. Regardless of why... it's best to ask.

Moving forwards, my advice would be to say something along the lines of: 


  As I've said previously, I'm sorry for resubmitting this presentation
  without your consent - I misunderstood expectations for how this sort
  of thing should be handled. I've withdrawn the poster, but I get the
  impression that you still have concerns about this issue. Is there
  anything else that you'd like me to do to remedy this?