Can they charge me if my THC was legal hemp?

Possibly, and it happens. An officer at the roadside has no way to distinguish legal hemp from illegal marijuana, so the arrest decision gets made without that information.

What resolves it is laboratory analysis measuring actual THC concentration against the statutory threshold. Where the result comes back below the line, the substance was legal and the charge shouldn't stand.

Two complications worth knowing. Not every case gets tested — smaller possession cases sometimes proceed on a field test and an officer's identification, which is a significant problem given what field tests can and can't establish. And documentation matters: receipts, packaging, and certificates of analysis from a hemp retailer are evidence.

There's a related and separate question of whether the search that found it was lawful in the first place, which is where Florida's odor cases come in.