My Scottish Fold is in a squatting posture; is it having an attack or just constipating in the litter box?

From one posture you cannot be sure the cat is sick, but the photo shows the hind paws cannot fully bear weight; if you feel swollen or deformed joints, it is likely osteochondrodysplasia, so get an X-ray to check the bones and treat pain.

When the disease acts up, the tail is stiff, hind-limb joints develop abnormally, leg bones curve, and paw joints swell; severe cases show toe deformity and inability to retract claws. Front limbs show similar but milder signs; the cat sits on its hips due to joint deformity, much like the photo.