I found out the hard way that there are two kinds of tahini, one without the shells and one made with the shells still on! The one with the shells on is terrible, it's bitter and tastes more like shells than seed. That's probably what you bought IE.
I found some tahini in my local publix, but am afraid to buy it because it is almost $6 for a small can and it doesn't say if it's shell-less or not. If I could at least see it I could tell but it is in a metal can. You can tell the difference by the darkness. The version with the shells is much darker, a little darker than peanut butter I think, while the good stuff is a very light tan.