Hello Juli. Welcome to Just Answer. Please allow me to assist you.
The P0751 code comes from two causes:
1) Low transmission fluid level
2) a faulty solenoid on the valve body (this is located inside the transmission oil pan)
A flush would not (usually) be responsible for damaging the transmission solenoid (Solenoid "A", to be specific), however, since the fluid was just changed, it is possible that the fluid level is incorrect. It is also possible that the trans dipstick (if it has one) is not in the correct spot to measure the fluid properly (they move if they get damaged). If your transmisison does NOT have a dipstick (some do not), the low fluid is indeed the problem. I would take this vehicle back to the shjop that serviced it and have them recheck the fluid level before you do anything else.
If you have any questions, please feel free to reply.
Thanks for choosing Just Answer. It was a pleasure to assist you today. I trust my input to your request is satisfactory to you and meets your expectations. A positive rating would be greatly appreciated for my assistance on your inquiry.
Thanks in advance and thank you again for choosing Just Answer