The inside of your throat is damaged, Radiation damages muscles - and the effect can be long term, and it also effects the way you swallow - it can also damage you epiglottis which helps prevent fluids and food from entering your lungs. This is all why a peg should be kept in place post treatment (plus the fact - as someone else told you - that rads continues to get worse after you've actually finished treatments)
Try thicker fluids this should help with some of the aspiration.
Hugs...