Does blood clots come out of the blue in young adults or are they followed by heart problems and etc?

Yes; it's quite possible for someone to develop a blood clot somewhere in their body and not be aware of it. The clot might not cause them any pain or any symptoms. If the clot breaks loose and travels through an artery and into your lungs or into your heart, you can drop dead without any warning.
Such clots often develop after an accident or immediately after surgery. That's one reason the nurses in a hospital make you get out of bed and walk the next day after surgery; or else you're given blood thinners after surgery - to prevent a clot from forming. 
If you DO develop a blood clot, your doctor will usually be aware of it, and will prescribe medication to dissolve the clot. 

Not usually, there is a reason, often there is a clotting problem such as protein S deficiency, or A fib, or varicose veins.  A blood clot in the lungs is called a pulmonary embolism, which you can look up risk factors for that, like hormones, long air flights etc. diet is not one of the risk factors for a PE

Relax. 

You’re far more likely to die suddenly and without warning of a stroke.

We all die, and there’s never any guarantees about total lifespan. So stop worrying about the random uncontrollable causes of death and just enjoy life, incorporating healthy choices to reduce the controllable causes of  early death.

Or, as William Shakespeare so wisely wrote in his play, Julius Caesar: “A coward dies a thousand times before his death, but the valiant taste of death but once. It seems to me most strange that men should fear, seeing that death, a necessary end, will come when it will come.”

the only problem you have is paranoia

get a check up and talk to the doctor about your fears. and maybe talk to a therapist about your fear of death.

https://www.blf.org.uk/support-for-you/pulmonary-embolism/causes

The biggest causes of blood clots is a bad diet coupled with too sedentary of a lifestyle.   My nephew is in the hospital with multiple blood clots right now.   He has two in his lungs.   Blood clots aren't necessarily fatal unless the go to the brain or heart.

This question has nothing to do with cancer.   It would be better off in other diseases and conditions.

funny thing...junk food [ lack of nutrition ] coupled with sugar drinks, sugar has zero nutrition and so steals what little nutrition is left in the body to digest the food ] that leaves even a larger deficit of minerals, the lack of which is the foundation for heart problems....we need therapeutic levels of minerals for good health......