I've been having sharp cramps on left side of my belly at 4 monts pregnant my home doppler device found baby'heartbeat on my pubic area, it was higher few days ago, near belly button, is my baby ok?does fetuses change positions going lower in uterus?

In the absence of any associated symptoms (e.g. nausea, diarrhea, fever, bloody urine) then the pain is mostly likely stretching pains as the uterus grows. Your baby is fine. Not only can it move around quite a bit in the early pregnancy, but where you happen to hear the heartbeat is very nonspecific and does not correlate very well to the baby's precise position.