One of the fun parts about traveling and speaking another language: sometimes my Spanish is really good, but the past few days people haven’t been able to understand me and asked me to switch to English. Maybe I got lazy, maybe my mouth is moving weird, but it happens. It comes back eventually. I hope
edit: the other part about it is that it does feel embarrassing, because I’m just ordering a coffee and failed to do that. Eh, whatever.
I think this happens at any fluency level. Sometimes I forget extremely common words in Spanish and I’m conversationally fluent. Sometimes my brain just flips to English mode and I lose the sentence structure somewhere between my brain and mouth. Best you can do is practice more until you aren’t thinking about it.