Thanks MP, point taken. I will nag her owner about changing the girth and ask her about getting her back checked etc. And I think she does enjoy being ridden, seems to anyway. I suppose as she had been ridden a lot the day before it is possible her girth area had got a bit sorer than normal...
I understand that this could be a frustrating situation for you because it is her owner who needs to sort it out. That is the downside of sharing. If the saddle and her back are OK, there are a few other things to consider:
Is she OK to groom and touch around her girth area? Some horses are simply very tickleish (is that how you spell it?) and don't like being brushed, touched or having their girth done up. Watch her face very carefully when you touch and groom around her girth area? Does she twitch her ears, move her head or body away etc?
Is the girth nice and clean? And is she properly groomed before you put the saddle on? Any dirt on her or the girth could cause irritation and make her feel sore.
Just before you get on her when the girth is tight, it might be worth pulling her front legs forward one at a time. This makes sure that there isn't any trapped skin between the front legs and girth area as this again could cause her to be sore.
Finally, when you do the girth up, make sure that you do one hole at a time. This will make sure that you don't give her a shock by whopping up the girth tightly to start with. I do Bens girth up in 4 stages. I do it loosely when I put the saddle on. I then tighten it when I have my hat on. I then tighten it again before I mount, and I tighten it again when I am sat on him.
Keep posting about how you are getting on. I love reading your diary. Your passion for horses and willingnes to learn is fantastic.