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Your trans probably has been neglected , and not properly serviced, oil changes and filter service. Try adding a can of chemtool B12, and operate the car for a sufficient time to get the trans oil real warm, then drain the oil, change the filter, and fill-er-up again, I would also put in another can of B12 at this time.

Sounds like it was neglected Probably didn't get serviced when needed And how long did you drive this vehicle with the issue before servicing it?That could have done it.BTW fellas AUTO TRANS DON'T TAKE GEAR OIL.The ignorance of some people irks my nerves if you know nothing of something don't push possible false info in peoples heads!Now Did you let the dealer service the trans or did you bring to an independent or do it yourself?Possible simple mistake if not professionally done best bet is to have it torn down to check for metal chunks and large metal shavings.I wish you the best of luck I will forever and always drive a standard do to the complexity and bull mess of automatic trans.

check your gear oil ... if it's the right amount then sniff it a bit and see if it smells burnt ... if it does then change the oil and transmission filter.

if you just changed your oil then you got the wrong grading for your car.

thats sounds like your low on transmission fluid.

check transmission fluid