Thanks so much for all your answers guys!

It's all really helpful.
I can't put my finger on what was different, Berkshire Mum. We did some sitting, some standing but that's normal for my lessons. My lessons are an hour but we were singing last night for twoat choir so maybe that was a bit of a shock. It was definitely my voice that felt strained though rather than support or anything else.
I'm singing with the altos which is where I've always sang and that feels very comfortable. I'm definitely not a soprano!! I didn't feel like I was singing loudly as I was so afraid of making mistakes!

(Definitely no belting, Rosfrog!!)
I think I maybe just forgot technique and was just singing along merrily and that strained my voice a bit. Maybe I was also concentrating more on getting the music right rather than correct technique, though I'm sure my technique is less than perfect, John. I'm still learning! I'll maybe focus on using the right configuartion more next week, Rosfrog, as I certainly wasn't consiously doing this last night.
We sing quite a range of stuff. We sang a song in Hebrew which was quite classical in feel, then a contemporary song and an African song.
It's so different singing in a choir compared to the stuff I do in my lessons and I've really noticed the difference this time!