Grumpy, Growly

That is going to be my pup-pup's new name any day soon. Lately she had been grumpy and growly and snarley. I'm not sure why. Anywhere she is laying down, sitting or you just go up to hug her and pet her, she starts growling and snarling. Now, she has always done this to a slight degree but now its gotten worse. Almost anytime you pet her and get too close to her "personal space" she starts up. I'm not sure what to do. I've started to say no very loud and stop petting immediately but then she noses my arm to be pet more, as soon as a pet more she starts growling again. So now I have lightly flicked her nose once or twice when she does it, and give her a very stern look. She apparently knows the "stern look" now cause when she growls I just give her the look and she walks into her kennel aka house and will stay there indefinitely (or until she hears food). But the thing is I cant coax her back out of her house. She stays in there, and I will sit outside of it calling her to come and she will take one look at me then "pretend" she doesn't hear me anymore and start to look away. I'm confused on what to do. Any help would be greatly appreciated.
And PS: the other day my boyfriend was getting too much into her personal space (and I warned him) and she nipped his nose.