sudden hardening of tummy

hi mummies, im at 21 weeks now and i realised past two nights when sleeping on my back my tummy will suddenly rise up abit and harden like its being pushed from the inside. is it normal? it will only happen once or twice and thats it. for around 10-20seconds. is that braxton hicks? i thought BH usually happens at lower abdominal only? how come my whole tummy rises and hardened :O