You are loving it to death. Lavender is not a fragile plant but no plant wants its roots ripped apart. You are growing INSIDE as a houseplant or as a containerized plant outside? They wanna be grown outside and can't even imagine growing them inside a house. Other problems I see, a "large pot" means a WET pot. Lavender loves heat, sun and will tolerant serious drought..., once established. Finally, I would not use an all-purpose potting mix for lavender as these products tend to be basically peat moss. If you can't mix up your own soils, get a decent mix for cacti and succulents instead. Sorry I could not be more positive.

Scarlett,

Good move repotting the lavender. A root bound plant is eventually going to cause issues. I imagine that it will take a while for the plant to recover from the root trim you gave it. I would also trim the leaves down to an inch or two above where the wood turns to green.

Does your indoor lavender get a lot of sun? It needs to get all day sun, outside is better if possible. A large pot is good for a plant, but you just want to make sure the water drains well, and don't water the pot when it's still moist inside the pot's soil.

It's normal for lavender flower stalks to droop during a hot day. This is how they preserve water which would other wise evaporate more quickly. At night the lavender should perk back up.