Do I have to inspect my clothes and wash my skin and clothes for prayer when a large amount of blood comes out of my back without me feeling it, given the great difficulty involved?

If blood exits your body by itself every day, even once, and without any action on your part, then it is excused for prayer, even if it is a large amount. This is due to the hardship and difficulty of avoiding it, though it is recommended to wash it off. Jurists have explicitly stated that impurities such as blood, pus, or serous fluid emanating from boils, burns, sores, and the like, that constantly exit the body or are present on clothing, are excused. This is based on the rule: "Hardship brings ease." This exemption ends upon recovery; once recovered, it must be washed off.