Hemoglobin, the protein in red blood cells that oxygenates the blood, provides the body with energy, strength, and stamina. When there isn't enough oxygen in the blood, you feel tired, weak, and sometimes faint or dizzy. It's hard to catch your breath, or you may feel pain in the chest when you try to breathe. Extreme fatigue can bring with it memory or concentration problems, leading to mental confusion. For more information, see Understanding Low Red Blood Counts.