I think it's the muscle. The expander was placed behind the chest muscle, it's not happy.

Yes, that what my doctor said because I am having spasms. Feels like little flutters inside the chest but these things do hurt.

I feel it too. I tightness sometimes, then a relief. Doctor says I'm healing.

If the tissue expanders are in pockets of the chest muscle its actually the chest muscle contracting.

Have you had radiation?