How accurate/good are EKGs at accessing heart problems, by themselves ?

Hi, I m 26. For more than a week now I ve been having chest pains, often it would hurt to breath, I would feel really light headed, and sometimes I would start to feel really warm and nauseous in my upper body, then after a while I d start to feel very cold and get goosebumps. I went to an InstaCare yesterday. The InstaCare doctor did an EKG on me, which lasted about one second, and said that my heart seemed fine. He gave me some solution with malox and some other stuff to drink, and came back about 5 minutes later. He asked if the pain went away and I said most of it, but some of it was still there (it was only light discomfort in the first place when I went into the InstaCare). He said that it s probably heart burn, and gave me a prescription for Prisolec (Omeprazole), and told me that I d only really need to take it for one week and then I should be good. He also gave me a list of foods to avoid and lifestyle changes for heartburn. I felt quite lightheaded after walking out of the InstaCare yesterday, and still feel quite lightheaded today, but at least the chest pains aren t very strong as they were before. Not to sound paranoid or anything, I was wondering if EKGs are pretty accurate by themselves? When they ran it at the InstaCare, it only lasted about a second. Since I m 26, I m probably okay, but sometimes you re just curious after visiting an InstaCare and they run a little test.