Yeah, this is a "noob" or stupid question whatever blah blah blah.
The other day, I played a local acoustic show. I have never ever in my life bothered with playing acoustic live until this show, so I ran into some problems. I don't have an electric-acoustic or an acoustic pick up so I figured I'd just mic the guitar. However, whenever I put the mic up to the sound hole, it would let out this loud deep feedback. Is this common? Why did this happen? Is it my guitar or is it the mic? What is going on? Is it stupid to try to mic an acoustic?

Like I said, this was my first time ever playing an acoustic outside of my bedroom (I usually do electric). I am seriously thinking about getting an acoustic pick-up, but even if I do, I would still like to know why the feedback happened.

put the mic and guitar away from the speaker
either the guitars strings where vibrating from the speaker or the mic was picking up the sounds and amplifing it