English lavender, Lavandula angustifolia - which it looks like you have - will not bloom all summer no matter how it is pruned. Generally, the bloom period starts around the beginning of June and lasts for about a month or so. Since yours is blooming so early, a light trim of spent flower stalks may generate a late summer rebloom but no guarantees.

Here where lavender is grown commercially, growers prune back the spent flowers in late summer/early fall and lightly take back about an inch or so of the foliage. In spring as the plant starts pushing new growth, you can prune back harder to shape and clean up and that should produce multiple flower stalks for the coming season. Just avoid cutting back into bare wood.

next time.. choose the pot with the least amount of flowers ... instead of the prettiest one that is already at peak bloom ... that way you might squeak a few extra days or weeks of flowering ...

and no recently transplanted plant should be in full blinding sun.. until the roots have time to settle down and start pumping water ... move that pot to shade for a few days or a week ... [your facts arent exactly specific on that.. so im speculating a bit]

ken