Very common for newbies to feel like there are problems like you describe due to STRESS. Stress can manifest itself in lots of different ways. Try to just relax and continue to practice. I assume having heartburn isn't something that you get often. As previously mentioned, eat a reasonable light meal before diving to help calm the stomach and digestive tract. Most importantly though, just try to relax and know that the regulator will give you all the air that you need. Focus on just breathing normal, just like you are doing now while reading this post.