Diggle's headaches are connected to the box he came across in the "Arrow" series finale, the contents of which was implied to be a Green Lantern power ring.
A picosecond is one trillionth of a second.
In 2049 in the pre-Crisis universe, August Heart was played by Kindall Charters, while here post-Crisis in the present 2021 he's portrayed by Karan Oberoi. As Nora West-Allen has yet to meet the present-day version, it's unknown if this change was merely due to the unavailability of Charters or if there's an actual in-universe explanation.
The episode marks Diggle's first appearance in The Flash since the events of "Marathon". Iris also returns after being absent for the past two episodes, as does Deon since his last appearance in "Family Matters, Part 2".
When Barry explains why he can't risk running to the future to check on Nora, he says that Diggle would understand the consequences more than other people. When Barry created Flashpoint, an unintended side effect was the erasure of Dig's daughter Sara from existence and the creation of his son John, Jr. in her place.