the bump on the back of your neck could be a number of things from an enlarged lymph node
to a sebaceous cyst. these are really common and not too serious but I would encourage you to get it looked at by your family doctor. enlarged lymph nodes in this are commonly caused by scalp lesions such as eczema etc. if you have dandruff too then this could be the cause.
your are very young to have a serious heart problem. as unlikely as it is I think it's important that you go to your doctor and have some tests including an ECG / EKG and maybe a 24 he tape where they monitor yor heart rate
. when your heart is beating fast this is called a palpitation
and this can be due to an arrhythmia
I.e an abnormal heart rhythm and this needs to be checked out . having an abnormal heart rythm can result in chest pain