Question: Until a few days ago I was performing my istinja in the shower after having briefly wiped myself with a tissue. Today, I fell in the same trap, as at the last minute I needed to go the toilet(to pass stool and urine) and I was getting late for jumu’ah. First I wiped with a tissue, when I was washing myself in the shower, I saw the colour of the water change to brown from the remaining faeces) as I washed my private parts. Similarly, the odd urine drops were still coming out.
Directly under the shower head is a plastic shampoo bottle. Whilst washing my private parts water inevitably went on to the bottle. I say inevitable because I do not go around monitoring these issues. It is also likely that during the rest of the Ghusl process water lands on the bottle again. Again I can’t be sure. Furthermore, when I turn the shower off, the head is positioned such that some of the remaining water in the head must drip onto the bottle.
My wife usually uses the shower after me and handles the bottle in order to use the shampoo, as she did today. She must obviously wash the shampoo off. How effectively or to what extent she does that I don’t know.

Answer: As you have yourself acknowledged that this is a trap from shaytaan, and you are suffering from Waswasah (neurosis)

If someone carries on this path of thinking then his neurosis will take over and shaytaan will use this to stop him from praying his namaz.

Until you can see impurity on your body then assure that you are paak. With regards to these Masa’il Galabah Zan is enough.

There is a maxim in jurisprudence
اليقين لا يزول بالشك
Certainty is not dispelled by doubt
So as it is certain for your body to be paak then just by mere doubts you will not become impure.

Similarly this will be the ruling for those items in and around the bathtub, that until any najaasat (impurity) is visible then the item will be pure.

May Allah swt cure from this beemari of waswasa, and give you relief. Ameen