I wouldn't bother filling the tires with nitrogen. They're already filled
with regular air, which contains about 78% nitrogen.
Yes, I think you have a leak, and since you're riding a bike that has
spoked wheels (a 2008 Nighthawk 250, right?) I recommend you install a
fresh innertube and after finding and removing the offending object that
caused the leak.
On spoked wheels, the rims often have sharp bits that press against the
innertube. It's the reason you're supposed to install a new innertube
every time you install a new tire. Look for those sharp points and file
them down while you have the tire off.
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