Yeah... tightening that nut changed the slack or preload on the bearings, and that is not for the inexperienced. Bring it back to the shop (preferably without riding the bike) for them to readjust. Sounds like the outer locknut was not properly tightened against the bearing "cone" and came loose (reason for the shop to correct this at no charge), accounting for the wobbling, but you tightening it without knowledge can cause damage to the hub. The shop will adjust it properly. Afterward, at your leisure, you can read up on "bearing adjustment of cup and cone, quick release wheel hubs", along with regreasing them beforehand. My suggestion is looking for video on Park Tool website, and possibly written instructions on Sheldon Brown website.