Thanks everyone, in the past few days, it's become more clear that she is favouring her back left leg , so I'm getting someone out to look at her 🙂 Thank you all for your advice !
I agree with Mariia here. Check your tack fits well and is comfortable for the horse.
Biting the bit and putting her head down is her attempt to escape the pain in her mouth. It is her way of communicating to you that she does not like the bit and the pressure it causes. It is not her being naughty - it is simply her way of letting you know she does not like it.
Due to this, I would definitely not recommend a stronger, heavier bit, since this will cause her to be more resistant and will create more problems.
Either change your bit to a much lighter, less harsher alternative or go bitless.
She may be in pain, ask your vet to check her over:)
Try a stinger bit and hold back more
Try a roller bit! Then she can't bite and hold it.
I would suggest you to try to change your bit to another one and see if you have more control. Also, it may be due the change of her food or the saddle. I personally had similar problem when I've changed my saddle, and because my position in the saddle was different the horse didn't like it and started to cantering away. Hope it was useful, good luck!
I suggest that you try riding with your hands higher than usual maybe even higher than ur belly button. And if she tries to canter off with you hold your hands higher and higher and work on small circles with lots of bending. Good luck!