Soon after brushing my teeth, i've noticed and very quite often i will get a dry white patch off skin within the exterior of my bottom lip. why?

Several reasons: You dont chew tobacco, do you? You could be having a contact dermatitis, or it could be film from the toothpaste. It could be something else all together depending on whats happening. Does it come off when you wipe it? How long does it last. Your best bet is to seek dentist consult to diagnose what is happening. Only then can you know what it is and what, if anything, you should do.

Sloughing: It is not unusual to have some skin slough off or peal to form thin film of tissue, pink or white. It is very common with certain toothpaste additives or trying new toothpastes or gels. Most likely not serious but you can let your dentist evaluate it for piece of mind.