Has your horse had a layoff from injury or perhaps just a little winter or R&R time off? If it's been more than a month or so, and sometimes even if less than that, it's highly likely your horse has changed shape.
Have you ever had a thin shim inserted under the front or back of your saddle, or a small handful of flocking added to rebalance it at a saddle fitting? You'll no doubt have been astonished at how much difference just a couple of mm change in the fit can make. Well, guess what? Your horse is on the other end of that change. You think that he/she hasn't changed shape? Do you think you could actually spot a change that is so small?
Best case is that you already own a shim pad and know what to do to compensate, and that the saddle you have can indeed be shimmed to correct the fit. For example in most cases a too-narrow saddle should not be used at all let alone shimmed. You're also a great rider who is sitting really well in the current saddle and can facilitate your horse to put on the correct muscle under saddle.
Shown above is a Mattes correction saddle pad, a shim pad, showing the stitching around the pockets, two on each side, and velcro opening to the side of the spine. These come with 3 thin felt shims for each pocket (more can be purchased cheaply) that can be contoured to support the saddle in a much more exacting way than with many generic shim pads that come with foam inserts.
Worst case is that your current saddle is a disaster, your horse has lost topline and may be more crooked than he/she had been, whether caused by injury or simply by loafing around in the field.
So how do we tackle this?
I usually recommend arranging a remote check at the point you're about to get on board, see:
https://stephaniebloomsaddlefitter.co.uk/blog-and-resources/remote-saddle-checks
In almost all cases this is best coming after a period of intensive groundwork, not just longlining etc for fitness, but some work to make sure the horse's posture is working well enough to carry you – that the thoracic sling can push up, and the hind end has "somewhere to go". You'll also have had your bodyworker out to give them the once over and of course vet permission to start ridden work if required.
I can assess photos of the saddle fit as long as I have seen the saddle on that horse within, at very most, the last year. We can decide whether it IS the right time to get back on, or if groundwork would be better for a period.
Once you've got on board and have started ridden work I would continue to offer help with shims and girthing until we do a fuller ridden fitting anywhere between 2 and 8 weeks later depending on how you get on. This saves the cost of two in-person visits, but only IF the basic fit is okay on that first remote assessment or can be made okay with adjustments that you can make to the accessories.