firstly you should have mentioned your age, also are you worrying about anything while on your back, this of course could start you getting anxious and your weight may contribute , l suggest if you are on your back try a meditation method lying straight out starting with your feet tense them then relax them , next your knee's tense then relax , going on the way up to your head, while you are doing this remember to breathe in and out , if it keeps happening then speak to your gp, hope this has been a help.