Join-Up is a good way to get to know a horse. Maybe the horse was bored before and will be more active when something new happens. Observing the horse on the pasture will give an idea whether there is a physical problem. If the horse is speeding around outside and is lazy during working a physical problem is not very likely, otherwise the vet should check the horse.
Tell us what happens, will be very interesting.

I've taken horses hunting (not fox hunting) and that has been a great way to perk them up but if you do decide to do this you should know the horse resonably well as even the laziest of horses can be difficult to control when your galloping with lots of unfamiliar horses at new places.

Just keep your work varied, hack out somedays and school or do groundwork (ground work is very good at keeping horses interested and is another way of bonding, I like this book for groundwork: http://www.amazon.co.uk/dp/0715324411/ref=asc_df_07153244112363861?smid=A3P5ROKL5A1OLE&tag=googlecouk06-21&linkCode=asn&creative=22206&creativeASIN=0715324411 )

I hope this helps!

Thanks for all the help, and I will tell you the result when I've started to work with her.
HorseCrazy