I went to a house party at my girlfriend's sister's house. I work at 5:00 A.M., so by midnight I was pretty tired and a little tipsy. I should point out that my girlfriend is 26 years younger than I am, so needless to say her friends are in their "prime" years. I went to the bathroom, then noticed the kid's room was empty as the parents had sent the kids to their grandparents for the evening.so I laid down on the bed. About 1:30 in the morning I felt someone unbuckling my belt. I was in a deep sleep, so it wasn't until I felt a soft hand teasing my hard cock that I stirred. I heard a girls voice whisper, "it's just me", so I thought it was my girlfriend. The soft hand slowly stroked and squeezed my swollen member until it was at maximum readiness. To my suprise, I felt a hot mouth and soft, warm lips engulf my cock, and recieved an incredible blow job different from any one I ever had. I was trying to figure out why my girlfriend was holding out on me the five years we have been together. I enjoyed the oral stimulation and the way she expertly sucked my cock until I couldn't take it anymore. I whispered, "I am going to cum", which has always been the sign for my girlfriend to stop and continue stroking my cock with her hand. On this night, the blow job continued. I again whispered, "I can't hold it" as I shot what must have been a huge load of hot, white cum into the hot, wet mouth. The blow job continued until I was completely drained, and then some. I reached down to hug my girlfriend, but for the first time noticed the body was different. A little more petite, and a little firmer. My hand was pushed away and again a girl's voice whispered to me, "Don't tell T____ (my girlfriend's name)" and the girl walked away, opened the bedroom door and left. I lay there, still blissful from the most fantastic blow job I remembered in years wondering who the hell just sucked my dick.