An occasional episode of constipation is not cause for alarm.
Stools seem unusually hard.
There is unproductive straining. Veterinary assistance may or may not be needed. In either case, small amounts of mucous, gooey, or even blood tinged stool can be passed and there is a lot of pushing involved. Difficulty urinating can also appear as straining. The point is that if all you have noticed is straining, it may not be constipation.