What is the ruling on someone who knew something was forbidden, then doubted its prohibition, then remembered it was forbidden?

It appears that the questioner is afflicted with waswas (obsessive whispering/thoughts). The cure for this is to completely disregard it and turn away from it, to engage in the remembrance of Allah and what benefits the servant in their religious and worldly affairs, and to frequently seek refuge with Allah from Satan. What happened to the questioner is not problematic.