SI issue ?

[TrasaM]

Brief history.. 6 year old black cob..dales type. Has always had a slight click in his right hip the last summer stifle started to lock. Worked him as advised ; trotting uphill and lunging and he improved a lot but still had a slight issue on downhills.
Two weeks ago became very sore and was barely able to place left hind on ground so called vet who said it's his stifle playing up so repeat exercise as before.
He's been stepping short on the same leg since and although it has eased a bit in walk he is throwing his head up in trot and poking his nose out in front which he never used to do.
Yesterday he threw in a little buck which he's never done with me before although he's dumped his owner and another girl who rides him a few times.
He's very cautious about his footing on slippy downhills ( mud) make him especially nervous so he will try to rush it. he generally seems happier on firm even ground.
Anyhow, long story short he just does not feel right so I've got the back lady coming out tomorrow to check him over.
What I'm worried about us that the problem is originating in the sacroilliac area. Or is this a chicken and egg situation? Is the stifle problem pulling his SI out or has an existing SI issue caused his stifle issues.
Anyone with experience of this type of thing please advise.

 

[TrasaM]

Boy was checked over this morning and had his saddle altered because he's lost so much weight since November.
Lots of issues and got a good going over by back lady. Neck not right, back tender and SI tendon also an issue. Tense right shoulder too and definitely in pain from the SI joint.
Advice.. No riding for at least a week and Walk out in hand for mobility. We've had to corral him in the yard to stop him going into the mud which she thought was the main culprit for his current condition.
All very worrying and my friend, his owner, is on holiday so we can't / won't tell her until she gets back Monday.
So my instinct was right but how I wish it wasn't.