Is it normal to feel exhausted in second trimester?

Elle
I'm 15 weeks and I'm so tired! I've had a bad headache which makes me sleepy, and I had a long-haul 4 day trip over the weekend, but this tiredness is pathological! I slept for 12 hours last night and just nearly fell asleep on train to work. Help! Am I feeling this because I was lucky enough to feel no symptoms at all in first tri? I thought this was meant to be trimester full of energy!! Anyone else?