If Rebecca knocked everyone out, why was she also unconscious? Did she knock herself out along with her enemies?
Well if we consider Rebecca utilising Dials, especially Impact and Reject Dials, then it makes sense that she as well as the other contestants are knocked out.
Rebecca is arguably unmatched in terms of speed in this Block D battle (except Luffy, she hasn't been seen to be hit by ANYONE yet). It is entirely possible that she could have positioned herself in a way to absorb the power of the Block D contestants into her Dial(s) while she was dodging their attacks and unleashed it back onto them at a later point - possibly chapter 733. Combined with her speed and agility, Rebecca is capable of utilising Dials to take out the Block D opponents 'one after another'. The Dial(s) could be hidden under her gloves or in her sword or elsewhere, it doesn't have to be noticeable just like it wasn't for Wiper under his bandages on his right hand. Impact and Reject Dials absorbing the kinetic energy of others and using it against them fits Rebecca's fighting still quite nicely.
Discarding the shield before she entered the Block D arena could be a signal of her intention to utilise the Dial(s) in the Block D battle. If Rebecca was utilising a Reject Dial during the Block D battle as well, it becomes reasonable to assume that even someone like Cavendish was able to be taken out. If we presume that Rebecca was accumulating kinetic energy within her shield from all her prior battles, it becomes even more reasonable to assume that the big names in Block D were able to be taken out.
Wiper has already shown that a Reject Dial can be used multiple times, so it is not too unreasonable that Rebecca could survive multiple uses of a Reject Dial, especially with her father and grandfather being such strong fighters and Rebecca herself being used to the world of fighting (a Gladiator).
Sure Rebecca may have preferred not to hurt others directly or harm them, but when that preference is placed alongside the desire to protect Soldier-san, it becomes apparent which she would prefer to protect. Rebecca is more like the "Wiper" of this arc than the "Vivi" of this arc - while Rebecca may not necessarily have the power to match the other main players, she has shown herself to have resolve and will (enduring all the hatred she suffered for most of her life and remaining
unbroken through all of that).
After multiple uses of the Impact/Reject Dial(s), Rebecca could have dropped to the ground out of sheer exhaustion, injury and fatigued, but given how she has shown herself to be quite resolved and mentally resilient, she may have willed herself to get back to her feet - just like how Wiper did during the Skypeia Arc.
Sabo may have heard something similar to what happened in the Block D battle through a story of Skypeia Dials and their combative abilities he heard/read. Given that not many "Blue Sea dwellers" know about Skypeia let alone Dials, it makes sense why everyone, including Bartolomeo was puzzled at what just happened. It could very well be Bellamy who informs Sabo and Bartolomeo about what exactly happened since he has been to Skypeia before, if he didn't leave the Colosseum yet that is. Or Luffy and Bellamy's timing in leaving the Colosseum could be very intentionally if Oda-sensei does intent to have Rebecca progress in the tournament and compete with the other big names - the unknown of Dials coupled with her speed can make her a force to be reckoned with.
As for why Rebecca is in possession of an Impact or Reject Dial, that could all be explained through the Corrida Colosseum. We know that these Colosseum battles/competitions run regularly and that Rebecca has been "imprisoned" as Gladiator for quite some time in the Colosseum. Couple that with the fact that the "heart" of the underworld isn't all that far from the Colosseum, then you have a potential prize of a Colosseum battle/competition being an Impact/Reject Dial or any other Dial/dial-infused-weapon. Since not many people realise what Dials are it would be reasonable to expect not may challengers for the prize unlike the Mera Mera no Mi which people know about through Ace. With Rebecca referred to as "The Undefeated Woman", if there were a Dial or Dials up for grabs in the Colosseum battles and she participated, she would have won them.
Dials + Rebecca's ability to use "Mantra" (Kenbunshoku Haki) could make her a deadly fighter like those seen in Skypeia.