Exhaustion

Sarah • Babygirl born 10.10.15 Due with #2
I'm only 5DPO, so pregnancy signs should be non-existent, but I've been SO tired the last three days. It started Saturday afternoon & has been on & off since. I also have a had a lingering headache. Anyone feel this way before their BFP? It's my first month on Letrozole, so I'm not getting my hopes up.