Fungus resistant to antifungal creams?

Hi rob246, it might help to know that antifungals like Terbinafine or Ketoconazole are not fungicides, meaning they don't destroy fungus on contact (like e.g. chlorine based disinfectants). Instead, they interfere with the metabolism of fungi. That is why it takes time for the medication to take effect. 

If you have a chronic well established fungal infection then it might simply take a longer time to completely destroy all dormant fungal cells (spores). The absence of symptoms (like a rash or itching) does not necessarily mean that the infection is completely cleared. 

On the other hand, you might just have been reinfected as the area around the groin is the perfect habitat for fungus to thrive. Likely places where this can happen is the gym or a public swimming pool. 

You will have to start treatment again and you could extend prevention after treatment by applying medicated powder. Some powders come in a pressurised spray bottle, contain ketoconazole or other antifungals and are very easy to apply. They have the benefit of keeping the area dry and prevent chafing.  

You mentioned that you washed your underwear at high temp. You could do that with your bed linen as well and also add laundry disinfectant to the last rinse cycle (instead of fabric conditioner). Effective laundry disinfectants contain the fungicide Benzalkonium Chloride.