Why have I been charged a foreign transaction fee?

Excuse me, I'm a bit confused about the recent charges on my account. Could you please explain why I've been charged a foreign transaction fee? I've been careful to only use my card for domestic purchases, so I'm not sure where this fee is coming from. Is it possible that there's been a mistake, or is there something I'm missing about how these fees are calculated? I'd really appreciate some clarification on this matter. Thank you.