I am 28.had a colonoscopy and the doctor removed an 8mm polyp.the doctor said it was not an adenoma, but it was a sessile polyp. is this pre cancer?

Yes: Current concept even though all adenomas are benign if left alone may transform to malignancy over the time , so they are pre-cancerous , must be removed and need colonoscopy surveillance more frequently than normal population , just don't worry see your doctor on regular basis and under go procedure if advised.

Not certain: Sessile means it was flat. You need to find out was pathologic type of polyp it was.