Depends what you mean by sore, and where, but it sounds as if you are sore due to sweat or wet (in which case you're correct, this is effectively 'nappy rash') rather than having a saddle issue or a shorts seam issue.
Some people just have more sensitive skin. I am one of these too. Despite wearing expensive shorts and having a wonderful new saddle I still get sweat issues, especially if it is a very hot day/long ride and/or a wet and rainy ride. I am also susceptible to prickly heat and to heat/sweat rash under my arms and elsewhere when visting hot countries, so I think it really is just a senstitive skin issue.
Any sweat, and especially any rain, keeps the chamois pad wet, which in turn keeps you skin supurating in salt and road grit. I would suggest then, that if the latter is the case, a rear mudguard - even a clip on one, when it's raining can help. A more wicking pad may help too, so yes that probably does mean more expensive. Also getting off the saddle every so often (on descents is good) a and letting one's bum dry out in the breeze helps a bit too. If you do stop at a cafe anywhere, mid ride, you could try washing your bum. Just wiping it doesn't help as you just grind the salt and road grit in, but a gentle wash, (without soap) can help remove any salt and grit. Make sure you dry really well afterwards too. Wash and change post-ride ASAP too. Nappy rash creams or any cream containing lots of healing zinc, such as Sudacreme, is a 'magic' skin restorer. It is very effective after the event to heal the raw/missing skin. It could be used as a chamois creme too, but it is quite thick and white so you may want something a bit less messy. Many creams, including assos cream, are water soluable so sweat or rain will effectively wash them off. You need to keep re-applying it for it to be effective. I am not sure if it could actualy be making things worse in a situation where sorenes is caused by moisture - it may act as a barrier cream at first, but only for a while and then you could be just making things worse by keeping the area moist rather than letting it dry out.
I agree that less is more with regard to shorts padding. You are better off with a well deisgned pad that is contoured to the body rather than something thick that bunches up and causes chafing. A thinner pad will also hold less water (rain or sweat) against the skin. Assos 'mille' shorts have a thin pad, but it's very well contoured and the shorts are a snug fit so they don't move around when you're pedalling. I have found them good for rides of up to 600km without any chamois cream. I also agree about an extra rinse cycle on all cycling kit - if you're getting sweaty or wet from rain, whilst wearing it and there is any residue of detergent, it has the potential to cause skin irritation. We use Ecover liquid, which seems to be fairly gentle on skin anyway, but we give all our cycling kit an extra rinse as well.
