Now suppose I have some urine or some drops of urine on my cloth. But I do not know it and am not aware of it. Now, I go and perform my salat. Now, will my salah be ok? Or does it become qada?
If the person who prayers, doesn’t know that his/her clothes is Najis, and after the Salah figure it out that it was Najis, his/her Salah is considered as a correct Salah (prayer), but it is better to do it again in accordance with recommended precaution. Accordingly it seems that your prayers are true. For further info. I suggest you to read the reference as well.
The first think about najis (unclean, based on Islamic ruling) is that we know it from the smell, color, shape or taste. So if we don't know whether your cloth dropped by urine or not, and you don't notice about smell, color, shape and taste, so you must assume that your cloth is clean.