Have just found this helpful piece on the internet, wish I’d found it before!

Colic takes place in a horse when the gut becomes bloated as a result of a build up of blockage or gas. Blockage may occur when fecal material is impacted, or in the most grave type of colic, by twisting of the gut. Causes of colic in horses can vary greatly. Reasons for this condition may include damage to the gut resulting from worm infestation, sudden diet changes, stress and even wind sucking. Acute colic can lead to a build up of toxins in the gut or possibly fatal shock and maybe both.
Colic symptoms in horses can vary. If a horse has a mild bellyache he may be uneasy, you may observe him looking at his flanks. If a horse has a full blown twisted gut the horse will be in tremendous frantic pain. Most of the time, a case of colic in a horse will fall somewhere in between these two extremes.If the gut is blocked by fecal material, medicinal mineral oil is normally administered by tube into the horse's stomach. This may clear the blockage in the gut causing the horse to colic. Surgery is usually not necessary in these cases. However, vital surgery is generally necessary if the gut is twisted.

Fortunately, it wasn't twisted.