I'm only ~13dpo, but have been waking up nauseous for the last few days and battling bouts of nausea throughout the day; something that never happens during my cycle. I actually took a pregnancy test around ~8dpo because my breasts were sensitive and the nausea had already started.
Before I even had a + pregnancy test I felt a great deal of cramping and and occasional sharp pains. This didn't surprise my obgyn very much considering I feel intense O pain. She speculated that the pains are being caused by the corpus luteum.