There are, as a matter of fact, many heart problems a bird might have.
Enlarged heart or Heart Hypertrophy may result from an active bird being in a too small cage and relying on short bursts of energy rather than a more steady exercise period. Sometimes the short burst of energy is the result of a predator or other perceived threat that frightens the bird. This is what people attribute to the bird "being scared to death" . In reality, a healthy, well exercised bird that either has a good deal of free time outside of the cage or (some birds are not safe being outside of a cage) those with extra large cages or aviaries have significantly less of a chance toward this happening.
Fatty hearts also contribute to this - and an all seed diet contributes to a fatty heart.
Arteriosclerosis (thickening, calcification/hardening) of the arteries - are other kinds of heart/circulatory disease seen in birds.
Sometimes they throw a clot - and the only symptom noticed is the stroke-like happening or worse, sudden death.
Other symptoms may be difficulty breathing, labored breathing, especially after activity.
A well experienced vet may hear the irregularities in heartbeat with a stethoscope, however, not all heart disease can be determined this way.
The problem is that there are no reasonable treatments once many of these diseases are diagnosed.
The only thing we have at our disposal is (just like with humans) prevention.
A diet of mostly pellets and only about 15-20% seed (less as the bird gets older), a nice big cage with a lot of interests for them to explore and do (change the interior around on the first of the month every month to keep it challenging), plenty of fresh foods like orange colored vegetables (squash, carrots, sweet potatoes/yams) and whole grains.
You can see more details about nutritional options and even how to switch to pellets here 4AnimalCare
I hope your little one is ok