Hi,
From some of your symptoms, it appears like an yeast/parasitic infection. If you have any intestinal problem, like parasites, e coli or candida infection, probably solving that might help you.
Solace
It's not impossible, its been happening to me for the last 5 years. You'll notice it become "flatter" more than anything. Only advice I can give is lay off for as much as you can to preserve it.
I never heard of a penis head shrinking. If you're masturbating, try not doing that for a month to see if you see any improvement in the look of your penis. Maybe you're damaging your penis head with all the vigorous rubbing.