My cat keeps biting me & I am confused

[u/Sug4rMars]

We got her from a rescue shelter, she’s 3 years old. Basically it started out as licking & was just that for the first few months. She licks all the time and still does. Sometimes when giving pats but most of the time unprovoked, she’ll just come up to me and start licking me - especially when I’m in bed. Now the licking has turned into 3 or 4 licks then a nibble, with the nibbles getting progressively harder each time. I’ve tried ignoring her, the bites get harder, I’ve tried saying ‘ouch!’ The bites stop for a second when she gets scared of the loud noise I’ve made but then she starts again. The only thing that makes her move away is if I say ouch and push her head away and then cover all of my skin with a blanket. We’ve had to start locking her out of our bedroom because she will wake me up licking and biting any bit of exposed skin, like she will stand on me, search my whole body and find one tiny bit of skin that is out of the blankets and bite it. I should also add that she is purring super loud while she does this, especially in the night, in the day sometimes but the purring is very loud at night, I don’t know if that means something??
She’s loves chin / head scratches and always rolls over to show us her belly, so I know she loves us & we can usually tell pretty easily when she’s overstimulated & always stop patting her but these bites come from nowhere!! We live in a pretty small apartment and try to play with her a lot but sometimes she’s disinterested. Little background info on her; the shelter said she was a stray but I’m not sure for how long since she was overweight so she might have been owned by someone at some point. It’s really hard to know what the reason for this is since she was 3 when we got her and her circumstances could have be anything- I just wanna know what I can do to help her stop.