Totally normal. Nearly all babies do it. ((In fact... try it yourself on the couch if your don't have back problems. DON'T do it slow and controlled, but as fast as you can. We've been taught NOT to rock back and forth over years and years of "sit still" & "behave"... but if you can do it without cracking a smile or even laughing you're a rare human. Most can't go back and forth as fast as possible without smiling/ laughing. I have no idea why.)) It's also a comforting motion when done slowly. Most adults will only rock themselves in extreme distress. But next time you feel sad/ stressed out... TRY it. It's naturally calming/soothing. One reason why adults build rocking chairs.
It CAN, however, be dangerous. Mostly risk from knocking her chair over. So keep an eye on the chair to make sure she's not going to tumble it over.
A normal person can't give themselves shaken baby syndrome, so there's no worry about "rattling her brains" when she's doing it herself (similar to the way that a person can't choke themselves to death... the body has natural defenses against it that controls motion... people to choke themselves have to use a prop like a rope/tie/sheet/whatever, their arms fly off -reflex- at a certain point if they try to choke themselves manually).
