Here is a question, what if Sweetie Belle's Partner is not exactly Rarity but someone who had a feeling that it was Sweetie under the glamour and just decided to mess with her?
Interesting point, however - to be able to pull such a prank in the state of postcoital bliss, one needs to be around the level of Patrick Bateman or someone alike. Alternatively, it is all just a play, and SB's partner was playing a role during the whole interaction. Add to this clearly troubled reaction of Bloom's impersonator to SB's freak-out, indicating either genuine reaction - or long-play manipulation: a jolly prankster would try to sooth the sudden panic, inviting the prankee to join the laugh instead of continuing the charade beyond that point. Ergo, if what you are postulating is correct, we have someone extremely manipulative in action, not just random prankster deciding to pull a leg in spur of the moment simply because it seemed funny at the time.
While not impossible, I see only few in-canon characters capable and willing to perform such an act - most probable candidate being, naturally, Chrysalis - who, as we were hinted at earlier in the story, is behind the whole Masquerade, presumably with unspoken or secret blessing of the Royals. Though, I suppose, one of her changelings may be involved as a proxy.