Just sounds like your having a bad run, or your friends got so sick of losing to you they started cheating. Watch to see if the're dealing from the bottom of the deck.
Most likey they are just getting lucky, and i would stop worrying about it as it shouldn't be important who wins in friendly games anyway. I always feel bad taking money from friends who are just there to have fun and have no idea what they are doing.
My only tip is, if they call anything... don't try to pull of big bluffs as most rookies will just call you down with absolutely nothing anyway.