I became doubtful of haidh starting but didnt check until a few days later. Once I checked, I saw haidh, but Allaah knows best when it started. I need to keep track of the days of my haidh. Shall I count from the day I was certain of seeing haidh, or from an estimate of when it might have started?

You should start from when you were certain and you saw it. I highly doubt that you spent days and did not check or notice something that came out. Definitely, you had to go to the bathroom or something similar to that, and since there was nothing, you didn't notice it. I do not want you to fall into obsessive thoughts, therefore I have mentioned this.