Could be "green beer", beer which still needs a week or two for the yeast to "clean up" various by-products of fermentation. If it was less than three or four weeks between pitching and bottling, or less than three weeks between bottling and drinking, your beer will probably mellow out a lot with another week or two of conditioning time.
Could also have fermented too warm, which can give you fruity or spicy flavors; for a stout, "too warm" is probably higher than 70 or so. If you're not actively controlling the temperature, just the warmth of active fermentation is probably enough to put you over the line. These flavors will also fade some with time, but less quickly and completely than "green beer" flavors.