The Phoenix.
That planet eating monster was living inside Rachel.
(She later used the Phoenix powered time powers to launch a former mirror universe NAZI super train they had liberated from the Reich, through the multiverse to new random destinations looking for 616 in Excalibur's epic Cross Time Caper after they got "lost".)
He's where it gets sketchy...
If Rachel had the Phoenix inside her during Days of Future Past, then she should have had the power to win out a war against clunky 90s tech Sentinels, and moreso when she Took the freejacking phoenix inside her back tot he 1980s... Are there not now Two Phoenixes?
Moreso, wouldn't the Universe (Rachel's mother was Jean Grey, not Madelyn Prior (A Jean Grey Clone with a seriously weird backstory who fathered cable/Nathan Christopher Summers because Jean was dead and ipso facto it was already an alternate timeline.) now that it has two Phoenixes fulfilling the same cosmic obligations, if not too much cosmic obligation has some serious congestion issues?
**Sigh**
Eventually Excalibur (Captain Britain, Phoenix, Night Crawler, Kitty Pride, others.) got bored of all this Days of Future Past temporal junk just shitting on them from some way off parallel timeline that was never going to be their future, so they went there to dealt with it once and for all.
But considering the further sequels and prequels that kept and keep happening, I don't think anyone but me read those books.