My peruvian cereus is getting soft and a little yellow?

i bought a peruvian cereus a couple of weeks ago...i planted it in the ground and i watered it the first time after 10 days....then 2 days later it rained....now it's been almost 2 weeks since it rained and it looks a little pale and it is soft at the touch....what can i do? is it lack of water? too much water, shock from being planted? should i fertilize it? I leave in Arizona, maybe this information can help