I suspect it is in second gear and not starting out in first or up-shifting. That is "limp mode" which allows you to drive slowly to the repair shop without needing a tow truck. The place to start is by reading the stored diagnostic fault code(s) in the Transmission Computer. Most inexpensive code readers won't access that computer. They only read the codes in the Engine Computer.

As a general rule, if it starts out right away in second gear, it is usually a sensor or other electrical problem. If it starts out in first gear like normal, then bangs back to second gear during or right after an up-shift, it is usually excessive wear in one of the clutch packs. That causes slippage that you don't feel yet but is detected by the computer.