It may have been too root bound, and when you repotted, suffered transplant shock. Did you try to unravel the coiled up roots (it's actually good to do that).
Also, Citrus does better with a special mix, rather than regular potting mix. It likes well-draining soil, so a bit more sand or perlite, and a little on the acid side. I mix in composted pine needles (nothing fancy, just dig down under the pine tree to the flaky stuff under the fallen needles) and small composted pine bark chip mulch. I also add compost, greensand and kelp meal for micronutrients.
Keep it in somewhat shaded location -- I prefer dappled shade under a tree or a shrub until it recovers, and not in hot direct sun. Eastern or Southeastern exposure to limit the sun to the gentler morning sun, then shade in the afternoon works too. Once it perks up, start gradually giving it more sun until it's back in full sun or mostly full sun (younger seedlings tend to prefer less sun -- they normally grow in the shadow of the big trees).