Mo
When I was at my worst 7 years ago (medical issues yet again!), I was 54kg (8 and a half stone). I am 5foot 6inches with a heavy bone structure. Anyone looking at me usually underestimates my weight by around 2 stone. I had dropped to a UK size 2 on the bottom half and UK size 6 on the top. I looked thin, gaunt and pale and even I could tell I was ill. I didn't need the hospital/GP/friends/family to tell me, it was very obvious.
When my weight eventually went back up to 60kg (roughly 9 & half stone) I was still a size 4-6 bottom and 6-8 top, but I was fitter and faster than I had ever been, both cycling and mountaineering, covering the route back from Corrour to Rannoch Station in my fastest every time - knocking 2 hours off it without even trying! (the Corrour to Rannoch Station via Cairn Dearg and down the ridge is a test route for us at the beginning of a holiday to guage our mountaineering fitness levels).
Family and friends all tell me I look thin now, better than I did, but thin. They are all concerned that I can not buy jeans to fit in places like M&S - sizes don't go small enough, yet (until this month and it is unrelated to my weight) I have been great for the last few years, stronger, fitter, healthier and more importantly happier than I had previously been.
Your own body will tell you what is right for it. Your times on garmin, strava or any other set route over the summer will tell you what you need to know, and if you know that you are fit & healthy and tired only because of the exercise you are doing and happy with your weight, then don't concern yourself with it. If you are finding you are routinely slower overall, have less energy, lost conentration, going down with each and every bug that does the rounds at schools, your body is telling you something and you need to listen, but if you are fit & healthy don't worry.
As mentioned above, we see so much in the papers about how being "thin" is bad for you (wrong words in the media, it should be "skinny" not thin) and how it is acceptable if you are happy to be fat or obese - well why can't it be acceptable if you are happy to be thin and healthy?