Make sure the chest tightness is related to the med and not a separate issue cropping up.
Sometimes people can have the opposite reaction to the Benzo. According to med literature, it's supposed to be uncommon.
https://www.ncbi.nlm.nih.gov/pubmed/15460178 That said, many people on the forum report odd reactions or even opposite reactions to meds they've taken for long times during withdrawal. The underlying question becomes ,"when do I stop it?" According to my experience and what others report, do not abruptly stop a taper as it causes more problems.
The micro taper seems reasonable. You've done very well to come from 10mg in July to less than 5mg by November. Due to how long you've taken the drug, you may be "feeling the effects" of fewer receptors and lower natural GABA now you're at <50%. Body might need more time to pick up. Know you're motivated to move forward, but could consider slowing it down or even holding a dose for 3 days to see if things improve.
Even though your dose is small, perhaps you could try taking it 1/2 twice a day versus all at once. See if it helps. Conventional wisdom says Valium is long acting, why split a dose? It can make a difference.
Anti excitatory supplements are niacin (I use non-flush), taurine, and buffered vitamin C. Adding those in may ease you through this phase. Taurine needs to be taken with food. All can be used as needed.
Good luck getting this sorted.
