Nipple flow size

When should I go up a nipple flow? My little one is only 6 weeks old, drinking 4oz every 2 hours (formula) but has started nibbling at her bottle instead of sucking, also getting frustrated and squirmy during a feed and there are times that she sucks really hard. Sometimes she’ll only drink 2oz then wake up soon after and cry for more. I burp her every 1oz sometimes 2oz and she’s on gentlease so I don’t think it’s gas. She’s using Tommee Tippee size 1 flow.