When the present Barry went back in time to save his mom, the Barry from the future (the one fighting Eobard Thawne) tells the present Flash not to save his mom.
Why so? Is this because he had seen the flashpoint universe?
We have to understand first that in this particular scene when Barry stops himself there are 3 Flashes:
- Present-day Flash (our Flash, behind a door)
- 10 year old Barry
- Future Flash.
The Flash who is hiding behind the door is our present day Flash, who goes back in time to try and save his mother, but gets stopped by the future Flash who is the one that saved Barry when he was 10 years old.
We never get the answer of why future Flash stops present Flash, we can only assume that he got stopped because the future Flash knew it could alter the future and start "Flashpoint (event)". As we can see on YouTube:
There could be many numbers of possibilities as to why he did it.
Now, the series has 5 seasons and there is still no answer as to why future Flash stopped present Flash. There is only speculations and theories. But to give you an idea of how it all connects, I will put it below.
SEASON 1 FINALE
Future Flash stops present Flash from saving his mother.
SEASON 2 FINALE
Present Flash goes back in time to save his mother, he accomplishes it, but before he talks to his mother he can see the first present Flash who is hiding behind a door vanish to thin air.
SEASON 3, EPISODE 1
Present Flash asks reverse Flash to kill his mother, so Flashpoint can be fixed and his current timeline can go back to normal. In this scene, the present Flash who came to save his mother in season 2 finale disappears, same as the reverse Flash who was there to kill Nora.
So your question could have different answers: i.e: Future Flash stopped him so Flashpoint doesn't happen. Future Flash knew it wasn't the time to save his mother, and told present Flash to wait, so the Flash in season 2 finale could save his mother (being that both are the same Flash).
A lot of crazy time travel, so there is no definitive answer, any of the previous answers could of being right.
Basically in the flash infinite crisis there is a scenario where he does go back in time and he saves his mom. And from there a domino effect occurs and over the course of many years leads to the world ending. Barry from the future i am guessing knew that on different action could lead to another thus leading to bad consequences.
Why did future Barry ask the present Barry not to save his mom?
We don't know yet. Until season 2, the show-runners haven't connected this dot yet, so we can purely speculate about it.
Is this because he had seen the flashpoint universe?
Is there even a flashpoint universe in the Arrowverse? And the answer is still, we don't know yet. We can clearly see that they took inspiration from flashpoint and it does look like a node to it, but as we know, the Arrowverse doesn't have what it takes to reach there; they don't have access to all those Justice League characters for now. So we might see an altered version of that story.
Possibly Future Reverse-Flash would have killed present Barry instead of his mother, so not only would present Barry die, but future Barry would be erased from existence, allowing Future Reverse-Flash to take over. Also if Present Barry would have went in to save his mom and Future Reverse-Flash go to kill him, he could also go and kill young Barry, totally wiping The Flash out for good.