I don't know the exact medical term but my best guess is you definitely have some tissue damage. Yeah if you had a proper tear you would see significant black and blue bruising and I doubt you would have been able to curl 6kg. I'd rest for a week then go back very light without squeezing the muscle at all during reps and see how it feels. I'm sure there are other members here that could help more than I. Let us know how it goes.