Like many stocks these days, Celsius Holdings (CELH 4.64%) has gone worse than flat. Shares of the company gaining market share in the functional sparkling beverage market have fallen for six consecutive trading days, a 21% plunge in that time.A couple of factors have weighed on Celsius stock. Costco (COST +0.43%) introduced a line of energy drinks under its Kirkland Signature label last week, and rising gas prices could make consumers less likely to spring for unnecessary indulgences. The market itself has ...