This is often used when saying excuses. Shortened to もん in spoken Japanese. I really hate hearing people say this to me.
sorry, I often use もん... だって暑いんだもん。いらいらするんだもん。;)
I often hear this in the context of a whiney complaint. Several Japanese friends advised me that only children and sometimes women use it and that adult males rarely or never use it (depending on the person).
I would add that 〜もん is not necessarily always in the context of complaining. However, it is VERY cheeky; using this can absolutely project a bad impression, unless you really are among the bold, frank, and sometimes even careless. (For an example of usage among men, see the Japanese language version of Red Dwarf)