I started off with istihadha on 13/04 and normally my Haydh is 31/04. On the 10th day of istihadha the bleeding has become heavy should I still consider it as istihadha as I don’t know whether this is haydh or not please could you let me know as I feel my cycle dates have changed now and I don’t know when it will stop

If the bleeding started off as istihadha but then later developed the conditions of hayz then the new bleeding will be considered hayz.

Conditions of hayz are continuity of bleeding over three days such that you cannot say you are not pure.

So let's say that on 23/04 your bleeding had continuity such that you are not bleeding for three continuous days and there is no time of purity between, then this will be considered hayz. By continuity I mean that even if the blood is present in the vagina and not necessarily coming out and you can check this by inserting a piece of tissue or cotton into the start of the vagina and if between two checks it is always coming out with blood then it will be considered continuity.

However if there is no continuity then it will be considered istihadha, just the type of istihadha will change.

hope the above is clear for you