Fatigue, despite getting adequate sleep, can occur without any known cause and as long as it not prolonged is typically not a reason for concern. If you had a reason illness, such as the flu or upper respiratory infection it can cause excessive tiredness for weeks after the illness. However, if you continue to feel tired after several weeks of getting adequate sleep or experience other symptoms, contact your doctor for further evaluation. If the fatigue you are experiencing is more than "tired" but is extreme exhaustion, I would call your doctor and request to speak to him on whether or not you should set up an appointment sooner, rather than later.
No. Absolutely not. The worst amount of average sleep per night that you could possibly get would be 3 to 4 hours. there is nothing wrong with getting 8 hours of sleep and feeling tired for a few days. Look at me, I actually get (and I'm being honest here) about 5 to 6 hours per day for the past couple years and I'm 17. Me and you are perfectly fine.