Movable suggests something located in subcutaneous tissue rather than muscle tissue. Soft, smooth, solitary all ‘nice’ characteristics for lumps.
You say you’ve had a heart exam, so assuming cardio. is OK. The lumps are most likely benign—do some research on fibrous lumps/ fibrosis.
And research the diagnostic capabilities of ultrasound before paying for an expensive one.
(House was a comedy about incompetent doctors IMO)
A full history and clinical exam should always be done before expensive tests. - Do it yourself if you want. Start a list of pains and niggles, past illness, injuries, falls, surgery, trauma etc… Feel all over your body for lumps and thickenings, tender spots etc—in all sorts of positions. Positioning can affect the ability to feel some things.
It’s a long while since I used ultrasound (and it was an old machine for even back then), but bone reflects ultrasound so over the ribs imaging is commonly distorted as the waves are bounced back. (unless this ultrasound is capable of scanning to a depth of less than skin to ribs).
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A possible explanation:
Chest pain can come from muscle pain. intercostal muscles, rectus abdominis muscles especially. Can be sharp and stabbing. Can be tight or shooting.
The lumps are fibrous, benign thickenings in subcutaneous connective tissue over your pecs. Possibly developed ‘to take the strain’ when the trapezius muscles aren’t sufficiently supportive.
Promotion of my hypothesis/the best advice I can give:
Have a look at this anatomy: main muscles and think about how you use your body, how balanced and aligned are you? Do you have a full range of movement or do you have tensions and restrictions? (I’ve written a lot more but a few pictures and a bit of self-touching will get you started.)
Give it a bit of thought over some days, feel how you move.
If nothing else, you should be able to find the lumps more easily again if you do decide to get further examinations.