Could be a bad oil pump. How many miles on the car and do you regularly change transmission fluid? If not and the cars old and has like 50k or more miles on it, it could be a much bigger, more expensive fix, like a leak from the torque converter.
From the bit of research I did, it would appear the first safe thing to do is change the fluid but makes sure the filter is changed too. Just doing a flush and/or having it done poorly could dislodge tiny sludge and particles causing further damage and issues. Hopefully it's just as simple as that.