Fetal heartbeat in 8 weeks

Hi mga moms, patulong naman po kung sino naka experience nito. Last week I am 7 weeks and 5 days pregnant then yong heartbeat ni baby is 121 bpm. Pinabalik ako ng OB for repeat ultrasound after 1 week. Kanina lang po (after 1 week sa last u/s) 117 bpm nalang yong heartbeat ni baby..Nakakatakot kasi bat bumaba? Sino po may same experience sa akin? Sobra worry ko na po????