Neon tetras have very particular water requirements, so it might be good to test your water for any anomalies (and check against their needs). It is possible it is just egg bound, but I couldn’t say either way.
It’s hard to see from the pictures, but perhaps you could check if its bulge is as extreme as in
this thread. Additionally, you may find some help
here. If it is severe, you could try feed it shelled peas in case it is constipated (that thread also mentions epsom baths, but I have had limited success with these; perhaps another forum member could help with that, if you make a thread). Hopefully the bloating, whatever it is, subsides soon so you can have one less thing to worry about!