It's going to become apparent to them sooner or later that you did badly (if you did as badly as you feel you did). I personally would, in your position, tell them you think you didn't do as well as you'd like to have done, rather than pretending you did fine and then have the results come back at the 10% mark. It's a lot easier to admit you didn't do as well as you'd have liked, and that you will be studying much harder when you repeat (if you have to repeat them) to make sure you will have the best possible chance of passing next time out, than to be pretending you think you did fine and then to have low marks come back - that to me (if I were your boss) would sound like you weren't able to assess how well you performed.
However, I'd only discuss it with the people who you need to, rather than random work colleagues - ie discuss it your boss or whoever you report to that will be getting your results. You don't have to say you didn't study, just that it didn't go well for you on the day, but vow privately to make sure you are prepared next time so you don't have to go through this worry again! We all mess up from time to time, so don't be overly hard on yourself. best of luck, and you never know, you might not have done quite as badly as you think you have done!