Two possibilities to mention:
1. If your skin is tender to touch and burning is not affected by body movements and different positions, then it can be shingles (Herpes zoster virus reactivation in one of the spinal nerves). Nausea can be also from shingles. This causes burning pain going like a stripe from the spine to the front. Often (but not always) rash appears after some time. Neurologist would tell you, if there is any available painkiler for your situation.
2. Bulging or herniated disc or some other disorder in your thoracic spine (osteoporosis, arthritis..) could cause pinched spinal nerve. This type of pain would be affected by different body positions and would not be tender to the slight touch. Again, neurologist could say more.
Synthetic HRT is, as you’d expect, hormone replacement made from synthetic sources. But (here’s where it gets tricky), Premarin — which is made from a natural source (horse urine) isn’t technically considered a “natural” because it isn’t exactly the same as the estrogen that is made in your body. However, some individuals argue that the synthetics — particularly synthetic progesterone, or progestin — seem to be associated with a greater number of risks and a higher number of side effects than their bioidentical counterparts. ( source )