Hi, from the sounds of it your radio control might have mixing enabled, or a channel is reversed. However you said that everything used to be working, did you fly correctly it before?
To check this you should connect your APM to your PC via mission planner and then go to the radio calibration screen, then if you move your throttle stock you should only see the throttle channel moving, similar with the other sticks, only one should move at a time. If this is the case you might of accidently changes the model on your radio. However if its just one channel that is causing the problem then you might have to reverse the channel on your RC radio. If you let me know what radio you are using I might be able to help you do this if you dont know.