This answer contains spoilers.
Please do not read if you are not up to date with the manga (chapter 89 is where this answer is based upon).
This question is strongly related with the title of the first chapter/episode: "To you, 2000 years from now". Check this related post on Anime StackExchange, which I answered similarly.
Eren's reality, is sometimes distorted by the memories of the future and past inheritors of the 'Attack Titan' power.
Also keep in mind that there is no official answer, thus the correct answer (for now) would be "We don't know".
However, after discussing on some forums and reading some theories, some hints provide a common direction.
As we know, Eren obtained the power of the Attack Titan with the Coordinate Titan (also known as the Progenitor Titan) from his father, Grisha Jaeger. Ever since, he has been having certain "visions", or "memories" especially while fighting other titans. In the manga chapter 89, this becomes clear.
SPOILERS!! Images from manga chapter 89.
(Picture source: Mangastream Disclaimer: I do not own these pictures. They are only used to answer this question more clearly.)
Leading to Hange asking:
Which brings us to the second hint of this chapter. Eren recalls the time he was able to use the Coordinate Titan's power.
Focus on "everything connected".
Which brings us to the culmination of the chapter (and the 'answer' to your question):
SPOILERS! If you have not read chapters 87 and above, do not read further!
Whose memories are these, I wonder
TO ANSWER YOUR QUESTION
There exists a memory-timeline that is outside of our perception of time. That is, Eldians who have obtained (at least) one of the 9 original Titan's power, have access to the memory-timeline of all previous (and possibly future) Eldians who obtained the same power!! In simple terms, Eren has the 'Attack Titan' power and the 'Coordinate Titan' power. He is able to access his father's memories, and either alter them or combine them with his own memories. Thus, the visions he is having in the first scenes of the first chapter, might be a result of a distorted reality, caused by the memories that the future inheritor of the 'Attack Titan' power is inheriting from the Attack Titan memory timeline.
As stated above, this is all speculation because nothing is revealed yet.
I apologize if this raised confusion, but I am more than happy to clarify parts of my answer.