Automatic transmission leaking out fast

Transmission leaking. I added a quart of ATF and three days later the mechanic ran diagnostics and nothing showed up. Mechanic drained and filled transmission at 172,902 miles (nine days ago). While on the rack he showed me that the transmission was leaking, but was not specific about where. The engine valve gasket and axle boot cover were also leaking. The car ran fine for nine days, two hundred twenty one miles. Now at 173,123 miles the shuddering problem has returned and the transmission fluid is again below the bottom notch on the dip stick. Shuddering and transmission leak were the original problems. Should I take it back to the regular mechanic or should I take it to a transmission specialist to determine the source of the leak? Should I try the Lucas Transmission Fix solution? I know now that just adding transmission fluid is not going to solve the problem. Since the problem developed suddenly I am thinking a puncture to the pan or some loose bolts. Although after the fluid change the mechanic should have properly tightened the bolts or noticed a puncture somewhere. Am I facing a minor or major transmission repair?