Maybe you did ovulate around the 17th and then had a corpus luteum dip on the 22nd? My chart has a dip at 3DPO as well and when I googled it it came up as that. Apparently very common and no indication whether or not you have conceived. It was my first month temping so I haven't heard of it before. The dip is a result of a drop in estrogen due to the egg being released, and then the corpus luteum (a little hormone secreting structure in your ovary) takes over to produce progesterone which causes your temp to go back up.
And then an Implantation dip on 8DPO but that doesn't explain the dip at the very end.
Do you usually spot before AF is due?