Feed From One or Both sides

I'm a FTM and am kinda of unsure about this. I was feeding my baby girl from one side at each feeding and switching to the other side at the next feeding she's been gaining a lot of weight and doesn't seem fussy at all but when I went to a WIC appointment the lady told me I was supposed to be feeding baby from both sides at each feeding like one for the first half and the second for the last half of the feeding. I've done it one side per feeding for so long and thought it was right and now I feel like I've just been feeding her wrong for months.. I thought according to how you feed baby your milk would line itself out and I thought my body just picked up on her feeding from one side per feeding but now I'm unsure.. is it normal to feed her like this or are you supposed to definitely do part of each feeding on each boob? I don't want to make my milk mess up or anything or starve her.. she always seems fine from one side and will go a few hours before needing to be fed again, she's always happy and playful and takes her naps for about 30 minutes every 3-4 hours a day. Like I feel like she is doing perfectly fine and is super healthy but I'm a FTM so sometimes I wonder if maybe I don't know what I'm thinking and I'm wrong..

** I'm adding this in just in case it's helpful but my baby eats pretty quickly like around 10-15 minutes from one side each feeding and sometimes she'll suckle longer than that, the lady at the WIC place made it sound like she wouldn't be getting enough to eat since she eats so quickly**