Head/neck shaking while riding

3 March 2021
I have a 10 year old Welsh mare. For context she is generally a spooky but well-behaved mare who does anything to please. She is always quiet in the stable and will let you do anything with her. Also, a new field mate has been added to her field around two weeks ago.
Recently, I have noticed that she doesn’t want to be groomed or ridden and is agitated in her stable. She also hasn’t been liking her girth going on. While riding her, I have noticed that she is shaking her head and neck as if something is annoying her. She is also leaning on the bit. She is unwilling to work properly (not bucking or rearing, just being lazy). I have stopped riding her for the timebeing as I’m worried that there is something wrong.
I will obviously be getting her physio to check but just wanted to see if there are any suggestions on here as to what may be wrong?
Thank you.