In photo three it looks like the corrugated sheets are just butting on top of the concrete wall and not overlapping it (and this is how you seem to describe it), is this correct?. If this is so the wind will easily drive rain water back inside the shed and down the wall. You need to either create an overlap between sheets and wall, or seal the gap between sheets and wall. Personally I would create a ''good'' overlap as it's more permanent (either with short similar sheets or even roofing felt). Don’t underestimate wind driven rain.....
Plus 1 to this. Are you renting the shed? If so, I'd contact the landlord to solve the problem.