At 20 weeks babies still have a lot of room to move around, mine was head down but the midwife said baby would be in a different position within 10 mins :+)
Babies turn a lot during their time in the womb especially when they still have space to move around. My son was head down at 23 weeks, then he went back to breech at 29 weeks and turned AGAIN back to head down at 32 weeks lol. Your doctor will let you know if you have anything to be concerned about.
Perfectly normal, its actually better!!