i am an 18 year old female, average weight 5'7" 140 lbs., and i've noticed lately that whenever i lay down, like to go to sleep, ever so often my heart will feel like it skips a beat, then the next beat will be like a thud, then return to normal. it only happens once in a while. is this normal? it is only when i am laying down.

It's not normal, but it is very common, especially in people that use drugs, smoke, or consume lots of caffeine. A good day of hard work can cause them too. What you describe is a PVC, followed by a palpitation. If they become a big concern for you, I would see my doctor and get an EKG :) Here is a good description of what causes a pvc :
Premature ventricular contractions (PVCs) are when the lower heart chambers, called ventricles, beat prematurely before the normal electrical activation sequence of the heart has occurred. Normally, the upper chambers of the heart, called atria, beat first and then the ventricles contract. When the lower chambers beat early (prematurely), the entire electrical system of the heart must be reset. This causes symptoms of skipped beats (palpitations).

i got an EKG a couple months ago and it was fine. i smoke cigarettes and marijuana every day, I guess that is probably why. does it cause complications?

At least stop smoking the cigarettes! LOL. Cannibis is known to cause heart palpitations, and if you smoke it long enough, it will cause heart disease. I read that people over 45 that smoke marijuana, have a 5 times greater risk of a heart attack :eek:

Jack51, that is very interesting. I never knew what a pvc was. I use to have palps sometimes. I still get those pvcs you mentioned although i've not smoked since 10/8. I will see a new cardio in 11/10 but wonder ......