What is going on here?

I met a lady two months ago thru friends( lets call her Karen)We have been seeing each other every Friday and Saturday night,and in the last few weeks I have been staying over at her house on Wednesday nights,if you know what I mean..
Last week she called me at my shop on Wednesday morning and said that her older sister was coming to stay for a couple days.I had met Jenni( sister) only briefly and liked her intelligence and sharp mind. I arrived at Karen's and we all had dinner and then moved out to sit on the porch in the night air and talk. As the night progressed I found myself feeling increasingly uncomfortable about staying over while her sister was there.Jenni was pleasant enough, but I detected a mild hostility towards me that was confusing.
The thought of walking around in my shorts in the morning and bumping into Jenni in the hall was daunting. I decided that I should go back to my place at the end of the evening and leave the girls to talk and catch up. At about 11pm Jenni went to the bathroom and so I said to Karen that I would not be staying because I felt "uncomfortable staying this time -I will call you tomorrow morning at the salon" -my words. Karen exploded and accused me of trying to force her to choose between her sister and me .HUH!!
As I walked to my car she followed and hollered that I was selfish and so on. I had to drive away really slow because she stuck her head in the drive side door and ranted on and on.
Next morning she turned up at my store with a letter that she had "written at 2 am --could not sleep a wink.I was too upset" Her words ! The letter was full of blame and critisim of me and how I acted the night before, accusing ME of getting angry.I read it a few times to try to get a grip on this whole issue.I still do not see why she got so steamed.She is acting all lovey dovey now as if it all never happened, but it is chewing away at me and I do not get it.Something is not right here ...Help? Tweet