I'm sorry to hear you've found a lump in the middle of your chest and for the worry this is causing.
Unfortunately no-one here can give you the answers you're seeking but as this is playing on your mind it may be worth booking an appointment with your GP as they'll be able to investigate and let you know if this is any cause for concern.
Hopefully this won't be the case but it's always good to get any unusual symptoms looked at, if only for peace of mind.
