There is no way of knowing what this mass was.
Having read your other message with you having a period that came 2 days early would not indicate a pregnancy, albeit an egg may have been fertilized and not implanted properly or something was not right. These things do happen and it is Mother Nature's way.
Don't dwell on things, the pregnancy test, you said in your other post, was negative. Most of the symptoms that you said you experienced could easily have been premenstrual tension.