This is an automatic transmission, right? If so, there are lines that carry transmission fluid from the passenger's side of the transmission to the radiator and back to cool the fluid. There is a bottom section of the radiator dedicated to cooling this fluid. It's possible that the radiator is giving up the ghost but it could also be a leaking line.
It's also possible that the transmission was overfilled after it was rebuilt and the excess fluid came out the vent and/or the dipstick tube. If the transmission fluid level is correct (checked with the engine running and warmed) and the coolant appears clean and at the right level (should be just covering the fins in the radiator), then it should be OK to continue driving it on short trips until you determine whether the issue recurs.