The word stretch can be used transitively (with an object) or intransitively (without an object). So your question may be, "If I use it without an object, how does the reader know what is being stretched?"
An "object" of an intransitive verb can come from it's definition. Google Definition: "straighten or extend one's body or a part of one's body to its full length, typically so as to tighten one's muscles or in order to reach something". So, by substitution, "I stretched" could be reworded with the definition, "I straightened or extended my body or a part of my body to its full length."
Then your question could be, "But which is it? The body or some part? What type of stretching was done?" This line of questioning could go on forever. How intense is the stretching? How long is the stretch held? How many repetitions? Language like this is always ambiguous at some point. Ambiguity can be filled from context: additional language, the environment, or world knowledge. From multiple experiences, we all have a sense of what a "morning stretch" can mean, so in this case, the ambiguity could be resolved from the definition + "world knowledge".
Consider the following dialog:
Mary: "Last year, I read this article that a regular, morning routine of stretching and calm breathing is good for one's health. So I stretch after waking up every day - for 30 minutes. First I do my legs - you know, hamstrings, calves, ankles. Then upper body stretches. All the while, I focus on my yoga breathing.
Joe: "Oh that's nothing! I've been doing a morning stretching-and-breathing routine for 20 years."
Mary: "Wow that's great! What's your routine like?"
Joe: "After I wake up, I yawn and stretch for 10 seconds. Then I'm good to go!"
Credit to @DamkerngT for pointing out transitive/intransitive.