That exact same thing happened to me with a Welsummer chick! It had picture perfect markings for a female chick, but right around 4-5 weeks, it started getting really large, pink comb and wattles. Another week went by and black feathers started coming in on the breast. I thought I'd wait to see whether it crowed or laid an egg just to be sure, but right around 6 weeks the little bugger started this screeching noise and I knew he was trying to crow - then I was sure.
The females comb and wattles will stay pretty pale almost until they start laying, and their breast feathers will be a *very* uniform salmon color.
Your chick looks exactly like mine did that ended up being a cockerel. Fortunately, I was able to return mine to the feed store to re-home it. I exchanged it for a known female. I would love to have kept it, as the Welsummer roosters are very good looking, but my circumstances currently wouldn't have allowed it.
Good luck with whatever you decide to do!
