Your wheel wasn't knocked out of true by the lip going out of your garage. The likely cause is that the mechanic merely trued the wheel without bothering to check spoke tensions. Uneven tension will yield a wheel that can go out of true very easily, including from the forces requires to dismount and remount a tire. Or, it could have been that way for a while and you just now noticed it. I'd suggest finding a mechanic with a spoke tension meter and let him thoroughly go over the wheel. Keep in mind that if the rim was damaged, he'll have to balance between even tension and trueness. A wheel with even tension yet not perfectly true will be stronger than a perfectly true wheel with uneven tension. You may have to settle for a slightly untrue wheel in your case.