Tortellini salad clumped together

I had an office lunch and made cold tortellini salad which clumped together. I added a lot of a vinaigrette I made which the tortellini absorbed. I did add more oil before refridgerating overnight but it still came out gluey?

I followed the direction, I rinsed the tortellini in cold water after I cooked it. I did overcook it somewhat (I turned the heat off and forgot about it so it sat in water) so I'm wondering if that's the reason?

When it was cool I mixed it with the vinaigrette and added more oil a little later on.