Okay... I hear people talk about intrusive thoughts but I don't really understand what that means.... is it memories or bits of memories or just your mind telling you evil things?

In session this week, my T and I were talking about making friends and I said that I didn't know why anyone would want to be friends with me unless I could do something for them... T went off in another direction but in my head I was having this different conversation and I told myself why no one would want to be friends with me.... because you are a "worthless piece of sh_t".. but its not a phrase that I have ever remember hearing (although I have dissociative amnesia for parts of my childhood) or ever using... and its a phrase that has been stuck in my head since Thursday... Yet somehow it seems so familiar... so right when I hear it..

Could this just be me remembering something? or is this what people refer to as intrusive thoughts?