Bruce has no idea what just happened.

One second you were fighting Calendar-Man.... the next he shoots you with his new gadget and you turn 5. He has no idea what's going on, you obviously don't, being 5 now. So he ends the fight quickly (not that it ever takes long with Calendar-Man) and gets back to the cave and investigates the situation. A few hours and a headache later he finds out that, if he doesn't want outside help from the likes of Zatanna or Constantine (he would rather die), it'll take about a month for you to go back to normal.

On one hand, god, he missed how cute and small you were back then, on the other... he was getting too old for this. You were 19 and in college, with friends and responsibilities... and you had an interview scheduled in two weeks, god help him. He's gonna have to come up with an excuse as to why you're not answering your phone or appearing in public. At least you're cute. Still a menace, but he missed the tiny, less scarred and traumatised version of the menace. Who hid in his cape and shadowed him everywhere.