Peace be upon you.
Ayatollah Khamenei (may Allah protect him):
Simply finding blood does not invalidate your prayer.
Ayatollah Sistani (may Allah protect him):
If a person was unaware of the impurity and did not realize it until after completing their prayer, they do not need to repeat the prayer during the time nor make it up later, provided they were not in doubt about the impurity before starting the prayer or if they had doubts but did not confirm it. However, if someone is in doubt without having investigated, it is obligatory (as a precaution) for them to repeat the prayer and make it up if they find it after the prayer.
May you be successful in all good.