There are all sorts of people that have issues with canulas. I guess it's an allergic type reaction. Unfortunately with an Omnipod there is not a choice of switching canulas like with other pumps. Otherwise people with issues prefer steel sets I gather and certain ones can be better than others? I use the Omnipod too, but I don't have issues.
Switching your site more often sometimes helps, I know one person that has to switch daily even, otherwise she has absorption issues. Try to call your doctor to see if they have any suggestions?
Also a thought, have you tried to give yourself some shots of Novorapid to make sure it's not the different insulin that's starting to cause a problem? Some people become allergic to certain insulins too..