I have a lump in the center of my chest sort of between my breasts, but a little lower, and my doctor told me it is an "epi gastric hernia". He said it is direct relation to my diseased gall bladder (too hard to explain why here, sorry). Nothing too serious, but something to get look at and possibly operated on. good luck!
It doesnt sound heart related to me (if you had a diesese that would cause your heart to swell suddenly, you would KNOW and wouldnt be posting here), but it could be something like a hernia, or a problem with your gallbladder. In any case, I think this certainly warrants a trip to your local doctors office for a full workup.
Good luck.