Multiple small lumps under skin

Hi there! Posting on behalf of my husband—M26, 5’9”, 180 lbs:

About two weeks ago, he discovered a soft, pea-sized lump under his skin near the center of his chest; a couple days later, there were about 5 more in the same area. Went to a doctor soon after, who felt them and didn’t seem to think they were tumors, cysts, or lymph nodes, and later confirmed this with an ultrasound. The doctor thought that due to the location, it could be costochondritis and/or tietze syndrome, which we were told typically resolves on its own.

In the time since, the chest lumps have started to shrink, but he has since found several lumps of similar size/shape in other parts of his body, namely around his armpits/pecs, his abs, and his upper leg near his groin. We are confused about why so many are appearing at once, and in so many different places. He isn’t dealing with any other symptoms and if anything has actually been feeling better and more active than usual lately. And due to a separate concern a few weeks ago, my husband had a full bloodwork panel, along with an xray and CT scan, all of which can back totally normal. You can’t see any lumps protruding from the surface of the skin, and there is no discoloration or odd texture above them; they are all soft and movable.

Any ideas of what this could possibly be? Why are all these appearing all at once? We are wondering if it’s something like myofascial pain syndrome or rheumatoid arthritis, or some other inflamatory condition adjacent to costochondritis. My husband thinks the appearance of the lumps correspond to areas that he has recently stretched or excercise (for instance, lifting weights and exerting his pecs) but if so, what could be the cause of that? Are there other types of conditions we should be looking into? The uncertainty and strangeness of it all has left us not knowing where to begin. Hoping to get a referral to a specialist soon, but aren’t sure how long its going to take, and some opinions in the meantime would be really helpful.

Thank you!