My heart tends to skip beats with no apparent reason but i often feel this when i am exhausted or stressed. why?

Very common: Not surprising that you may notice it at times of stress. Caffeine will make it worse so refrain from that. Unless you're feeling lousy with them or you're having a very fast sustained heart beat (which is quite different than what you described) I wouldn't worry about it. They're a nuisance for sure. Heck, I'm a cardiologist and I had em and they drive me crazy but they're not dangerous.