what your trainer said is true but i would like to refine it a little bit. You need to take electrolytes with fluids. Your muscles tends to get fatigue
very soon as they lose water very easily, so to hold that fluid in your muscles you need some electrolytes also with fluid. You can get these electrolyte powder easily in any medical store.
As far as your old muscle injury
is concerned, i don't think that is causing this problem.
I hope i have answered your query.