How to know when my puppy is being aggressive?

I have a 5 month old jack russell terrier who whenever she's a bit sleepy tends to be quite aggressive. If she's lying down and someone goes over to stroke her, she tends to go all stiff and tense, even if she's not asleep, just resting. She looks you in the eye and sometimes makes a noise under her breath, the thing is we don't know if this is growling or not. It looks like she doesn't like us stroking her when she's lying down but if we stop then she wags her tail and turns towards us. However, my friend went to stroke her the other day and she growled, but my friend continued thinking this was nothing and she tried to bite my friend. It was all very sudden and shocked all of us. Surely a puppy shouldn't be like this? She hasn't tried to bite any of us since but we are all very wary of her now, especially because of the way she looks at you, like in the corner of her eye and the noise she makes under her breath. I don't want to just not stroke her when she's like this because that's alot of the time, i had a dog before who was as good as gold and even when she was sleeping we could stroke her, pick her up and do whatever and she never minded.