Take a pic if it's still around. Also, Green Frogs look just like bull frogs with minor differences, namely being a dorsolateral ridge, and will gladly populate your pond, especially if the water is OK. If their tympanum (ear drum) is same size as their eyes, it's a female (both species). If it's noticeably larger than their eyes, it's a male (both species). Both species have different, distinct calls though, making it easy to distinguish w/o actually seeing it. If you remember hearing it's call or if it's still around, watch a couple videos of each call for verification. Green frogs have various calls, but usually those videos are playing what you'll hear most often. Their eggs appear to be similar too.
From what I've seen around my pond (probably have 15+ green frogs, not including 25+ of their tadpoles just growing rear legs now) is the male (green frog in my case) will pick a spot and claim it. He'll eventually call out periodically. If challenged, it'll give a warning call. Eventually, females will come in to the area, or are already there, and have the ability to respond to the calls; I have 1 large female that's been lured in by the males and seems to hang around now (or will leave and return) and has laid eggs twice now (few days ago and earlier in the year). Guessing she's 3-5 years old. She'll go to the spot where he called from, then lay her eggs. Eggs "unfold" in about 3 days and they'll cling to nearby surfaces until larger. Have a bunch of smaller females and had a large male around, but he's either left, been eaten, or died somehow.
Anyhow, bullfrogs are considered invasive, and carry a fungal disease that can kill other amphibians, and can grow much larger than a green frog, and guess eat more too. They should burrow into mud under the water if they stay all season. In all honesty, hope you had a green frog.