Sometimes indigestion or heartburn can cause chest pain and a feeling of heat, plus of course an upset stomach. Try looking into that, maybe changing your diet or getting some sort of medication to soothe it. Other chest pain can be muscular. I myself recently saw a cardiologist for aching and occasional stabbing pain slightly left of center and that what it was. Try pressing on the area, on the center of your sternum, then on whichever side of the sternum your pain is on both on the ribs and in between the ribs. If it hurts when you do that, the pain isn't coming from your heart. If it hurts in between the ribs it's the muscle, and if it hurts when your press on the ribs themselves, it's probably the cartilage. There's not too much you can do about the muscle other than applying heat to it, which can help increase blood flow to the area and reduce pain and promote healing over time. You can also take anti-inflammatories like Advil or aleve for a while and see if that helps reduce the pain. Irregularities in the heartbeat can indeed be caused by panic attacks, anxiety, and stress and worrying about it probably isn't helping your situation! Try to relax and know that heart issues are taken very seriously by doctors, and if they checked everything and say you're ok, you don't need to worry about the cause of the pain too much. Whatever it is isn't dangerous.
Also, sometimes when I stand up my heart rate drops, and I get this tight or uncomfortable feeling