feeling guilty after a fling

so i just started seeing this girl but we're not official or anything. yknow? and then one day she texts me out of the blue asking if i wanted to hook up tonight. it was spur of the moment stuff really, we met at her place and did what comes natural. afterwards though...i felt awful, like maybe i should've waited until we were something more solid. or that i'm just a selfish guy who's not thinking about anyone but myself. do u think what happened was wrong? let me know if it's okay to feel this way.