Dont be worried! i had EXACTLY the same thing happen to me. I went in for my 20 week scan and they couldnt get pics of her heart and even her spine and told me that im going to have to come back in a few weeks to het a better look coz bub will be bigger. I went back 2 weeks later and even tho she was STILL un co operative we were able to get some good pics and there is absalutley NOTHING wrong with her heart. HTH. Im 1000000% sure ur bub is just fine!!