For God's Sake!

Some complete non-event at work is being completely blown out of proportion and resulting in what looks more and more likely to have me hauled up infront of HR.

I was covering the Museum patrol position and the patrol had arranged for this lady to go see the new wing between 1 and 1.30. Bear in mind that I'm still on the radio and supposed to go aid if requested and go to where I'm needed so really I shouldnt be doing this but its the only chance we're getting to go look in the new wing before it opens.
Anyone half one comes and goes and I get called several times on the radio but I can't leave my position. I'm getting more and more pissed cos I'm gonna be bitched at for failing to go, cos there's a huge group in front of me that really needs more than one person dealing with it and the girl I'm covering is recovering from having a cancerous growth in her leg so I try and do a lot of the pointless busy-body running about errands that Patrol does to help her out.
Come 1.45 its time for me to go and relieve somewhere else, and have patrol take over and the original lady, D - go on her break.
So I grab someone to hang there for a moment and scamper off to find my patrol and hand back the radio.
Patrol mentions that D has been in the messroom for a good five-ten minutes. This pissed me right off. It may only have been for a short while but she should have come back from the tour to her position and not go take an extra long lunch break.
So I'm seriously annoyed and storm to the staff room to confront her, and yes as her supervisor I'm well aware that it should have been done privately, but this woman is not afraid to lie, play the race game, and cry to get her own way (despite being the same age as my mother.) She's dragged several people to HR. So although wrong I decided it was best to talk to her in front of witnesses.
But i'm also not going in there hollering and yelling as I want. So I approach her and ask her why she didnt return as I and Patrol asked and she agreed at 1.30.
"Oh I didn't think it was worth it."
"I ASKED you to come back at half past. I had several radio calls. I'm not there specifically to cover you."
She suddenly flipped and started yelling about how she couldnt be expected to walk there and back again in half an hour. I pointed out that others managed it. I did her a favour I didn't have to do and I expected her to be back by half past as arranged.
She burst into loud tears and then tried to usher me towards the door demanding that if I had such a problem then why didnt we go see a manager.
I lost it then (frankly I don't know how I controlled myself) told her that she was pathetic in so many words, that she didnt need to go see a manager, that I as her senior had dealt with it and she needed to accept that she was in the wrong.
I then left.
I then get called to speak to the manager as D is "shaken by my verbal assault and needs to go home to recover." wtf?
Yes I shouldnt have raised my voice and maybe my judgement call of talking to her in front of others was wrong. But from her track record I felt it best to do so with witnesses. I certainly didnt go in there screaming and shouting and didnt raise my voice until the last minute.
Even if she didnt get into the wing until 1.15 as she claims, then surely common sense would have made her think "oh I'll just look at one part and then scamper back." or "i'll leave it for another opportunity."
And why she felt she could take an extra break rather than coming back is beyond me and its beyond me why I'm now suddenly in the wrong for this "verbal assault and bullying."
The managers believe it to be a non-event. I think they secretly agree with me. but she wants to take it further so its all very up in the air and I could be taken to HR for a disciplinary hearing.