I have never heard of anything like this and I can't find a source that mentions it. However, it's perfectly possible that an allegation like this would have been made and recorded somewhere. What you have to understand is that slander and rumormongering, particularly of a sexual nature, were very much part and parcel of Roman politics and, consequently, of Roman history writing. You might sort of think of it like modern tabloids.

So for example, another one of Caesar's mistresses was Servilia Caepionis. They were certainly sleeping together (it all came out in explosive fashion when she tried to sneak him a love note during a tense debate in the Senate), and it was widely rumored that Servilia's daughter Junia Tertia was actually Caesar's. These rumors got blown out of proportion to the extent that Cicero publicly joked when Caesar bought some property at a low price to give to Servilia that he was "getting a third off." (the latin word for third is tertia and the implication was that Caesar was porking Junia). The thing is, it's pretty unlikely for both of these things to be true - if she really was his natural daughter, the intense Roman distaste for incest makes it extremely unlikely he'd have had sex with her, especially if he was also still involved with her mother.

Caesar was also publicly accused of being gay, or specifically, of having traded sexual favors to an eastern King in exchange for being allowed to borrow his navy (Rome frequently borrowed or rented navies from other powers during the Republic). Modern historians think this is probably not true, but it was used as political ammunition by his enemies during political debates later in his life.

Now, as Mark Antony's mistress, Cleopatra was a very important political target in Rome for Octavian's faction when they had their contest for control of Rome's territories. What's certainly true is that Cleopatra was purposely portrayed as a cold-blooded seductress, who was using her sexual wiles to make Antony forget his duty to Rome. As a part of this, might somebody have remembered some rumor they had heard about Caesar's pet names for Cleopatra back from when she was Caesar's mistress prior to his death? Of course they might, or they might have simply made it up. Romans regarded performing (as opposed to receiving) oral sex as degrading and humiliating, appropriate for prostitutes or slaves but not otherwise, and so portraying Cleopatra as doing a lot of it or doing it well would be a very effective smear.

So it's pretty likely that this either happened, or was alleged, and that in either case the story was circulated for propaganda purposes by somebody who wanted to make Cleopatra look bad. But because this kind of scandalmongering was so common in Roman politics, it's often very difficult to establish whether any of these allegations were actually true.