It sounds like it is your D light is flashing from the description. Unsure, though. If it is the D light flashing, you have a transmission diagnostic code stored. Most common issue with your Pilot is a transmission pressure switch has gone bad, but we need to know what code is stored to be sure. Not dangerous if it is just that, but we need to know the code stored to be sure. I would recommend to get it checked soon.
Let me know if you have anything further. I'm here to help.