Is a very small amount of Istihadha considered Najis? If I discover that I had a tiny amount of Istihadha immediately after my prayer, does that render my prayer invalid?
Istihadha requires you to perform Wudhu for every single Salah whether obligatory or recommended. If the Istihadha has occurred before performing your Salah, your every Salah needs to be with a new Wudhu.
If Istihadha occurred immediately after you performed your Prayers,v your Prayers which performed are valid, but coming prayers need a new Wudhu for every Prayer. This is when the blood is little which does not fill the pad.
Wassalam.