Gum receded between two front teeth, could this be due to brushing or flossing? (Photo)

Unfortunately, it is very doubtful that this tissue will return. Grafting is a possibility but this is a very small defect so discuss your expectations with your periodontist prior to surgery. It may actually be easier to replace the veneers. 

Thank you for your question. The area between two veneers at the front, is quite critical for what are terms dark triangles. 
Your dentist would have warned you before treatment if it was likely that you would be at risk of having these. Some teeth are positioned in a way that makes it almost inevitable that this will occur. 
If your dentist planned to mininise the effect of this, ongoing hygiene maintenance (flossing regularly and seeing a Hygienist) would be the best ways to keep this fragile piece of gum tissue intact. 
Not flossing was not a good idea, and at the very front of your mouth, your technique would have to be very poor to be the cause of the recession.
As you rightly noticed, bleeding increased when you stopped flossing. 
Gum grafts are possible, very expensive, and for the minimal amount of gum to be increased unlikely to be a reliable procedure. 
It would be better at this stage to consider correcting one or both of the front veneers, to 'plump' the gingival tissue up slightly and make the visual appearance of the dark triangle less apparent. 
Redoing one would be cheaper and with a careful shaping, the asymmetry of doing it this way could be hidden.
Good luck with whatever you decide to do -
Thomas Dey, BDS, MSc