Exactly the same thing happened to me a few years back. I was on a full sized roundabout going straight over, clearly had right of way and was at the mid-point of the roundabout, then some dozy bint thinks she could pull out from the next junction in front, panics and hits the brakes. I had no choice but to slam on the anchors and as a result, skidded and hit her back wheel.
If she didn't admit liability straight away - at best would have been a 50/50 decision - but due to the stupid rules, it would be my fault as technically I hit the back of her car, even though I was on the roundabout with right of way.
You should have called your insurance same day to let them know