Home and able to put more time into a response here.
It might not be the Tena's causing a flare up, I am not aware of anything having changed recently so if you are using the same brand you have for a long time then it is unlikely to be the cause.
What may be causing skin issues could be something else on your skin, new moisturiser, soaps, deodorant, or any number of things from the environment around you not mixing well with urine, there could also be something due to changes in the weather and temperature (for instance being a bit warmer might mean you are sweating more and even just a little bit can cause a "jock itch" or chaffing rash) or a combination of things. Even down as far as the things you have been eating or drinking may be having an effect on your bodies chemical balance. Heck even increased emotional stress can cause rashes and other physical problems.
Then there is the matter of aging and skin deterioration over time, as you get older the skin gets weaker and thinner thus more easily irritated, this isn't an overnight occurrence and takes a long time but it happens, then there is the matter of personal hygiene, I'm sure you take care of yourself but maybe you haven't been quite as thorough recently and it's creating a problem, easy to do. Or perhaps you had a bit of a rash/infection that may have seemed negligible or to get better but hadn't fully cleared and so keeps coming back.
Considering the number of nappy users out there (from babies to adults) skin reactions to a specific brand of nappy are actually very uncommon, it can happen but not that often, it is also possible to develop an allergic or similar reaction to something that you have been using for a long time but again in the case of nappies this is unlikely.
Trying a new brand of nappies is probably the easiest thing to try and "rule out", and if it works then after a while try switching back to Tena's and see if it returns. but it may be something else that needs a different approach.