It's possible that your lemon tree suffered from lack of water while you were away.* Citrus likes to dry a bit before watering and then a deep watering is a good thing.
You say it's in a large pot and it's new.* I'm wondering how long it's been in the pot and if you transplanted it from a small one into a large one.* Generally when repotting, the new pot should be about 2" larger then the pot it came out of.* If it was rootbound and you didn't tease out the rootball, it could be having problems taking up water.*
Since this is a young tree, expect that you will have some bud drop and fruit drop until it matures.* It's common with citrus.* You should find this helpful.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.