I have the same "damn, I used to be good, now I'm not" with the 'cello (formerly grade 7). And indeed with the piano (formerly grade 8+), as observed at aiwendel's when after barely passable renditions of a couple of numbers I decided that it was the better part of valour to hand over to deborah_c for the big fast chords in "The Phantom of the Opera". :-) You can get away with more flaws on the piano, especially if people are singing at the same time, but there are limits.

The ear is exquisitely sensitive to even very slight weaknesses in technique, especially if given years of training. I don't think musical instruments are quite so much unlike riding bicycles as they seem; it's just that a slight wobble when you get on a bike for the first time in ten years doesn't make you want to clap your hands over your ears.

FWIW, when you picked up the violin and started playing, of course it wasn't a particularly brilliant sound right at the start, but it's fairly easy to tell the difference between a rank novice and somebody who used to play competently. In the latter case, rather than complaining, it's usually best to assume that they can hear the flaws too and let them get on with correcting them! You were getting noticeably better throughout the evening.