You know I reckon this is pretty common in babies of that age, my DD also did it and I also found it pretty darn baffling (and sore)! It only lasted a few weeks then she stopped doing it.
But I don't think it is 'just' playing - there may be a good practical reason she's doing it, as this sort of behaviour can actually speed up the milk flow by stimulating your breasts to let down more milk. Maybe she is just going through a growth spurt and wants more milk, faster!
Sorry I can't advise at all but will watch with interest as my boy is doing a very similar thing - clamping down with his gums whilst pulling back to have a look around at whatever has caught his interest! It's as if he just wants to make sure he doesn't lose his place while he has a quick look around! Quite cute but nor very comfortable and a bit worrying how it'll progress when the teeth appear!
My LO (20 weeks) tends to do this more when snacking, and less when he's really hungry. I am now trying to stretch him a bit longer between feeds and then doing it away from TV & people/distractions (or under a muslin). When he's hungry, there tends to be less messing around.
I'll also change sides...as for me he does it more on empty-ish floppy boob (sorry of TMI), whcih makes me think that theboobmeister is right and he wants more milk, faster.
My DS does tend to look at me fr a reaction and whilst there no way in hell he'd understand "no" at this age. Not rewarding him with a smile or laugh seems to make him look a little crestfellen and not repeat...immediately anyway .