My Iphone 11 ( just bought is brand new a few days ago) is getting a little warm while using an Apple 20watt fast charger, should i be concerned? it gets cool again once i take it off charge but i’m concerned about my long term battery lifespan.

I've experienced similar issues with iPhone 11 Pro Max and the 20W power adapter. I don't think that quickly charging your iPhone will significantly shorten the lifespan of the battery. If your device gets uncomfortably warm while using the device while charging, unplug your iPhone to let it cool down.

https://www.apple.com/batteries/maximizing-performance/

Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures - Apple Support

If you're concerned about charging your battery and its impact on the lifespan of your battery, make sure that you have Optimized Battery Charging enabled. Optimized Battery Charging learns from your habits and limits the iPhone to 80% until it's necessary to charge the device further.

About Optimized Battery Charging on your iPhone - Apple Support