I had the same thing - the partial denture was too tight and it also pressed against the bony lumps and bumps which are on my gums where teeth had been extracted over a period of time. It broke the skin in two places by rubbing. I went back to my dentist and he checked the spots where they were pressing by putting a type of 'carbon' paper (plastic it felt like) between the denture and my gum and it marked the spots where it rubbed. He then ground and polished the denture so it didn't 'ride' on any high spots.
I had to go back 3 times and he said that was normal. Some people go back more times.
It still bothered me so I resorted to adjusting it myself with small pieces of emery paper (but I don't recommend that ).
Mine also seems to change depending on the temperature in my mouth - sounds crazy but a really hot or cold drink seems to make alter a bit. Some dentists refuse to use the flexibles (Valplast ?) because they do cause problems this way, but I'm getting used to them.
Persist with your dentist and keep going back until they are OK. You paid for them and everybody goes back a few times.
By the way I only wear mine to eat or if I go out somewhere because I can't get used to them. I'm sure they are better than the conventional plastic ones though because they are so thin and light. Boog luck and remember 'persistance pays off'.
Going back to your dentist to have denture adjustment is advised.