Thanks for help everybody :)
I'm sorry for the bad video, I took 5, but only that had a horse in it
Mom came with me today and we went to our neighbour's round pen (I didn't know if neighbours are comfortable with me taking videos on their property, so I didn't)
We think she is lame on both hind legs, depending on which direction we're going.
She also stands like this a lot: IMG_4732.jpg
She was always standing like that when she was on a steep hill, but in the last month I noticed it's more and more often. She stands like that when she's resting, eating, when she stops, even if I rode her and stop, she would put her leg like this immediatly. She doesn't prefer any leg.
And about 2 months ago, when it was one of that rare days that I went to my neighbour's round pen to lunge her, she was having problems with keeping correct lead in the hind when cantering. She started right and got a wrong lead in the hind in the corner then fixed it and lost it again...