When you push too hard when you poop you can cause damage to your intestines and anus as well.
Pushing too hard during pooping can cause and lead to anal fissures, or tears in the thin, delicate mucosa (tissue) around the anus.
That can then cause bleeding of the anus and you may notice blood in your poop as well.
If you need to strain to poop then you're likely constipated or have a bowel blockage that is partially blocked or you have some other colon or bowel issues.
You should never have to really strain to poop when you're normally healthy.
Poop should easily come out without much if any strain.
Sometimes I will feel like I need to poop and then I will go sit on the toilet and then it takes some effort to start the poop going.
If that happens then I don't force it and then just wait for awhile longer and then the poop comes out easily.