It does seem like a pressure issue if all gears are slipping, and there was no issues before. What was the condition of the oil before you drained it? Check the filter, make sure it's not clogged up. Especially make sure no clutch material is in there. If it is, well you know what your problem is.
Sometimes if the tranny fluid deteriorates, then the next time you change the fluid it kills the clutches because of the high detergents in the fluid.
