Am I experiencing a flatline?

Either way it's good, because you're staying away from the porn and your brain is adjusting. Flatline is usually a combination of no sex urge, tiredness, depression, anxiety, and a general lack of motivation to do much of anything. It's also possible you're starting to feel what it's like to not see porn all the time. When you're not seeing it so much, you don't think about it so much, and the urges kind of evaporate - gradually, and with occasional spikes of crazy urges.

For me, the first week or two were the hardest by far. Before that, it was just about white-knuckling through the urges. After that, the urges were less, and the challenge was not getting overconfident, because then I'd get blindsided by a sudden urge later on and not be prepared, or I'd assume I was doing so well I could risk something iffy like an HBO show and gradually fall back into porn.