I have a hen, not sure of breed, that hatched 2 eggs
on mothers day. I have had her and the 2 chicks in pen by themselves since. Yesterday she was missing - found her in their little house - sitting on 16 eggs. Of course they weren't fertile, but she acted like she wanted to start sitting again. Is this unusual ? I wasn't expecting her to start laying again for a couple more week much less start sitting again. Can anyone shed some light on this ?

Chickens set whenever they want to. If you can get some fertilized eggs somewhere, you can put those under her. She will have something to hatch, instead of just sitting on unfertilized eggs.

It isn't normal, but I've heard of chickens doing that before, sounds like you have a very determined girl. I once had a chicken that sat on her eggs, but when her first chick hatched, she was so afraid of it, she ran away and we had to take it away before she would come back on. She hatched one more chick and then sat on her eggs for 2 more weeks before I finally moved her outside. She threw a big temper tantrum and went and made a nest in the garden and found some rocks and sat on those! Put her good mothering skills to use. Like Anonymous says: As long as she's setting, get some fertilized eggs she can sit on. Good Luck, I hope she hatches out some more beautiful babies!