It definitely sounds like anxiety, I get exactly the same thing you describe, even when I feel like im not worried, it still happens, it feels like my heart is fluttering or beating really fast/hard randomly. Ive been having it for a few years now and I know its anxiety. The more you worry about it happening, the more it will happen because youre anxious. Try to stay calm when it happens and take some deep breaths, wait for it to pass and remember its only anxiety, you are fine and nothing is going to happen to you. The fact that you have jjst started a new job can definitely add to it, the stress of a new job can make anxiety worse, thus causing palpitations. Just relax and remember you are fine, eventually they will happen less and less.